I think we need to be able to discuss things in the commit phase the way we could discuss things in the definition. I mean the discussion above all the questions. That is needed to keep the community spirit in the commitment phase. Of course, the commit discussion should be separate from the define discussion.
I think by "discussion," you're really asking for the ability to comment on proposals after they are in the commit phase, which is already the topic of this post:
I'd suggest to open the "meta" site early, for the commitment phase. It could also be helpful to start a sense a community. And even it may allow people to organize a promotion campaign, to propose already written "promotionnal messages" that we can post on appropriate forums (I often hesitate to promote because I'm not good at "selling" things and I'm not sure I'll explain well what it Stack Exchange and why people should like it).
For instance, in "Guitars", I was looking for someone that has a high reputation in the Ultimate Guitars community. So far I contacted random people but most of them don't like the principle of creating another community in another website. Even if I explain that the aim is different some of them feel we want to steal people from their community.
That's also why I hesitate to post a promotionnal message on their forum. I'm not sure I'd be welcome.
So, I'd have been really happy to talk about it with the people that will be in my community but so far I can't easily communicate with them !
So I'd vote for a meta site as soon as the proposals are in commitment. Moreover it will give already committed people something to do in order to work for the building of the community.