A scheme with selling proprietary license requires a unification of the copyright. Who should own the copyright, me, FSF, or all contributors? I would prefer to avoid such problems by making donation voluntary.
If you issue the proprietary licenses, *you* need to have permission to do so. Either the contributors assign that right to you (by granting you a license, for example) or somebody else (FSF(E)) can be the copyright fiduciary but would still have to grant you that right. The author remains the copyright owner, in most cases.
I would not think this makes sense in such a scenario. If FSF denies you the right to hand out proprietary licenses, then what?
Mirko.