The difficulty with reverse-onus provisions is that we're speaking of them and trying to apply them to specific circumstances. The Crown absolutely has the ability to make arguments for the detention of someone, whether they are the one who bears the onus or the accused person bears the onus.
Some of the difficulty that then comes in is that, when we think in a principled fashion about the difference in power between the state and an accused, if we're talking about denying somebody's liberty, perhaps it should be up to the state to make those kinds of arguments.