Much as I hate prolonging a probably-pointless discussion... I hope we aren't thinking about keeping things difficult for everybody simply because everybody includes some people who may want to take advantage of GCC in a proprietary way. In the long term, this only benefits the folks you'd be trying to exclude.
Think about it. You have nothing to fear from people writing trivial add-ons (if they're useful, they'll be duplicated in open source; if not, they'll fade away). The only people you might need to worry about would be those writing significant new compiler designs/enhancements using GCC as a starting point (and possibly trying to get past the GPL by avoiding direct linking/etc.). Yet as has already been pointed out, they already can do this by creating their own interface to plug into. If they use a standard interface (since the code base would favor this), it would be easier to replace their plug-ins with open-source alternatives later. -Jerry