> The problem with opt-in is that people won't use it unless they (a) know it's available, (b) know how to get it, and (c) actually go get it. So people don't really do that. But establishing an opt-out convention like this solves the problem entirely, and it's simple.

I don't see any of those things as "problems". IMO, it should be opt-in, on a per-project basis. There could be another checkbox in each project's "Settings" tab. The default should be that people are left alone.