As The Cat and I have been discussing in the comments here, the sets of tags between Stack Exchange sites need to be kept separated. Unfortunately, that is the exact opposite of what's going on with the proposal site -- it's a discussion area for all proposals. Therefore, any tag system would need to be completely segregated by proposal. I think this could be implemented, but it would be quite a lot of work.
Secondly, and more importantly, I don't think a tag system is even necessary. There shouldn't be a need to categorize the questions of a proposal if all the questions are of high quality (as Robert Cartaino details here). For example, how many example questions about Starcraft are necessary? If there are that many, then maybe (a) the lower-quality ones should be removed, and/or (b) a new Starcraft-based proposal should be started.