From what you've said, I think the way forward is apparent. As you surmise, accepting GPL v3 contributions isn't possible with the current project status saying the project licence is v2. Actually, I think you COULD accept v3 contributions, but to do so you'd need to change the project licence to v3....Or to v2+, if I understood correctly.
No. In that case you're granting permissions that the author didn't grant. Think about it ...
You're given a load of code that is licenced "v3+ plus OpenSSL exemption". You then put it into a v2+ project ... BIG NO NO. You've just gone and told all your recipients they can distribute as per GPL v2 - something the v3 author did NOT give you permission to do!
You said that your authors at the moment are a bit chary about moving to v3, but you think it's a good idea. What's actually probably a good idea then is to say that "All new contributions must be v2+ or v2/v3, in preparation for a move to v3". (Or BSD, or some other GPL-compatible (both versions) licence.)
That doesn't alter the project's current v2 status. It DOES stop a developer throwing a spanner in the works by contributing some new v2-only code which will prevent you from relicensing. And it makes clear to developers where you are planning to go.
Cheers, Wol -- Anthony W. Youngman - anthony@thewolery.demon.co.uk