I wonder about one thing though: you mentioned some people who were uncomfortable with this situation, and I believe you that you really talked to such people over beer. But aren't some other people perfectly happy to contribute to a codebase which might be additionally relicensed under a proprietary licence?
Whenever people contribute work to my project -- Tahoe-LAFS -- I ask them (a) can we please have it under the same terms that we distribute Tahoe-LAFS under (which are Free Software and Open Source), and (b) can we please also have a license to use it however we wish including proprietary relicensing. Also I tell them that we will immediately release their contributions under our Free Software/Open Source licensing terms as soon as their contributions hit our revision control repository.
Everybody so far has said yes to (a) and all but two contributors so far have said yes to (b).