IANAL, but that would essentially require contributors to sign a waiver correct? That seems like a fairly large barrier compared to just being able to take contributors code and incorporate it into a closed project, no?
All major corporate open source projects have a contributor license agreement (CLA) which contributors must sign. Regardless of whether they intend to close the source in the future or not. See the VLC license change fiasco for why CLAs are very good: http://www.jbkempf.com/blog/post/2012/How-to-properly-relice...
If it's GPL or MIT, then it can be forked, and the original owner can't force anyone to sign a waiver. The waiver idea only works as long as the original owner maintains control of the de facto distribution of a project.