The steering committee mailing list is actually open to everybody to post. So on the technical side, I don't see a problem. On the social side, though, I see more difficulties. It will tend to create more barrier of entries for authors to make successful proposal, for instance (one more thing to get subscribed to, I should also mention that I've noticed that people in their twenties tend to be pretty uncomfortable with mailing lists). So I think a hybrid discussion where the committee speaks among themselves, then come back to the author, is probably the best option. It does put some more work for the shepherd which needs to organise the back and forth. But hopefully not too much.

Other than that, I agree with everything Richard says and proposes. I'll add one argument: there is no good reason why the public discussion on a proposal should stop just because the committee is having a chinwag about it.