Nice! Having finally grokked the layer mechanism magic, this sounds like a great idea.
I know I suggested this for 1.5 along with the hooks, but I realized after that I really need the hooks more than pluggable functions - so far, I can't think of any functions I would have overridden, just ones that I would want to cancel (events, mainly).
A set of functions I could see editing are those that output html, e.g. qa_post_html_fields(), but that can also be done in a layer, I think.
Actually, this would probably be a good way for a plugin to modify the recalc functions, no? Suppose we wanted to check for extra points, we could just slip into qa_db_users_recalc_points() and... well, first I have to grok how that works :) But yeah, all the recalc functions would be nice... oh, and now that I think of it, qa_recalc_perform_step(), if there were some way to add an $operation for a totally new recalc function, that would be handy... maybe that's too much for 1.5, though :)