I'm fairly sure that - legally speaking - this is meant to come from the cabinet. There's historic precedence for the Assembly "asserting" a power by... well, doing it, and the legal authority for such an action being implicit in the assembly then passing the motion/action/whatever by sufficient a margin that it would, in the form of a charter amendment, grant the assembly said power.

I'm not sure how well that would stand up in court these days though.