In the "war on drugs," the presumption of innocence is not collateral damage; it is a deliberate target. If other people plant drugs on you (e.g. to avoid being busted themselves), you must prove that your possession of the drugs was unwitting (which you can't). This situation is manifestly incompatible with the rule of law, because the power to convict -- the most fearsome of all government powers -- is effectively taken from the courts and given to those who are willing to plant evidence. There is no clearer case of a "government of laws" being usurped by a "government of men."
In the few jurisdictions in which a person claiming unwitting possession faces only an "evidential" or "evidentiary" burden of proof rather than a "legal" burden of proof, the key question is whether an innocent person would necessarily be able to adduce sufficient evidence. As rules about the burden of proof have evolved in order to deal with a foreseen lack of evidence, it would seem that the answer is negative -- in which case the breach in the rule of law remains.
The purported reversal of the onus of proof, being contrary to the rule of law, is unconstitutional for at least three reasons: First, the mere existence of a constitution, written or unwritten, presupposes the rule of law and therefore invalidates any legislation or judicial precedent incompatible with the rule of law. Second, the mere existence of a court presupposes the rule of law and therefore precludes the court from entertaining any proposition incompatible with the rule of law [to compromise the rule of law would be repugnant to the institutional integrity of the court.] Third, the legislative power is limited to the making of law, which, by definition, must be compatible with the rule of law. Legislative provisions purporting to reverse the onus of proof, being incompatible with the rule of law, are not law and are therefore beyond the legislative power.
Wherever there is an overriding prohibition on cruel or unusual or degrading punishment, there is a fourth reason -- namely that the reversal of the onus of proof foreseeably leads to the punishment of innocent people; such punishment being cruel or unusual or degrading by reason of their innocence.
Therefore, if you are on the jury in the trial of a person charged with possession of a prohibited drug -- especially if that possession is construed as trafficking -- and you are told that the "law" requires the defendant to prove that the possession was unwitting, it is your civic duty to uphold the real law: put the onus of proof back where it belongs (on the prosecution), raise it to the proper standard (beyond reasonable doubt), and hand down a verdict on that basis. If the verdict is "Not Guilty," the acquittal is binding and no further action can be taken against the defendant -- or the jurors.
Defenders of the status quo will object to the fact that when drugs are found in homes, vehicles, baggage, or outer clothing, the possibility of innocent possession is always there. From this, they conclude that the accused must always be convicted. The correct conclusion is that the accused must always be acquitted. Defenders of the status quo take the position that if the State can't get the outcome it wants without reversing the onus of proof, then the onus of proof must be reversed. The correct position is that if the State can't get the outcome it wants without reversing the onus of proof, then the State must not get the outcome it wants.
How then shall the drug pushers be defeated? Not by starving their business model, but by constipating it.
To discourage use of drugs, we need retail ("street") prices to be as high as possible. To discourage the trafficking of drugs, we need upstream ("wholesale") prices to be as low as possible, so that concealable quantities are not valuable enough to be considered worth trafficking. A bottleneck in the supply chain -- e.g. due to law enforcement -- raises prices downstream and lowers prices upstream. Therefore, law enforcement should be concentrated on the retailers.
To avoid sending the wrong price signals, enforcement further upstream in the supply chain should be just strong enough to maintain the need for concealment [the risk of confiscation, or of prosecution if caught in the act of selling, is enough; the risk of prosecution for mere possession is too much]. To encourage the retail customers (junkies) to inform on the retailers, the customers must not be at risk of prosecution for possession or purchasing. When every retail customer is a potential informer with immunity from prosecution, nobody will want to be a retailer. There's your "bottleneck."
To meet these requirements, the supply of prohibited drugs should remain an indictable offense, but possession or purchasing of any quantity should be a summary offense punishable solely by confiscation of the contraband, with no conviction recorded (so that prosecution would be possible in theory but pointless in practice).
The same arrangement would neatly solve the problem of unwitting possession. If the drugs are yours, confiscation involves a loss. If they're not, it doesn't. Either way, justice is done. And notice -- especially if you're a politician -- that the arrangement does not amount to legalization or an end to prohibition.
The legislators could further strengthen the deterrent against retailing by creating an offense of being a "habitual" supplier of drugs, so that a seller could be convicted on the evidence of a large number of buyers, each of whom was the sole witness to a sale, without the need for two witnesses to any particular sale.
Thus, it is not helpful to prosecute people for possession, let alone to reverse the onus of proof. Indeed, reversing the onus of proof protects the bosses of the drug trade. If the bosses arrange for the drugs to be in the "possession" of some unsuspecting person, and if that person is caught, that person takes the rap and the investigation never reaches the bosses.
But of course the legislators, in the scramble to be seen as tough on crime, don't mind if the wrong person gets convicted, as long as the median voter doesn't know. They don't mind if the drug trade keeps flourishing, as long as they keep scoring political points by pretending to fight it. They'd rather keep punishing innocent people in defiance of the rule of law than admit the need to pay compensation for past unlawful "convictions," as long as the day of reckoning doesn't happen on their watch.
So, if you are charged with possession, and if the prosecution can't prove that your possession was voluntary and you can't prove that it wasn't, you should invoke the "rule of law" defenses (and, if applicable, the "cruel or unusual or degrading punishment" defense). As the prosecution won't want those points of law to be tested in court, threatening to test them might persuade the prosecution to back off.