Couple of brief thoughts Rob (aside from the slack idea, will respond later to that too):
I would suggest sticking with the discipline of simply bringing up proposals as the fastest way forward to more efficiency along with stuff like using asynchronous proposals, and not necessarily "pitching" proposals ahead of time per se... though if the proposal is very complex and you want to give people a heads up, I see nothing wrong with that.
Or if you want feedback, there are no rules against sharing a proposal in advance for feedback, or asking some questions to a colleague, etc. I think it could role model (no pun intended) something that could look like or create a pull to consensus, and even ignoring governance records and formal authorities, depending on how you do it...
Also, hold proposal crafting sessions in pairs or groups, and do more governance meetings so that people are comfortable with the process.
In HolacracyOne, I would estimate that the majority of our proposals are done via asynchronous proposals in GlassFrog, and since they aren't typically urgent tensions, any that are escalated or are usually just brought to the next meeting. Some are simple, and some are more complex. We have a lot of experience crafting proposals and with the process, so I think that this more efficient expression of Governance comes later, after getting comfortable using the concepts and practice first.