I'm willing to work on this (though my available time is somewhat limited). I think a good first step would be to make individual modules available for download and install separately (perhaps via monger?).

Sounds like a workable approach for a quiet migration to externality. Especially if it would (as I presume is the idea) not impact from what source (in this case the main Pike repository) the modules end up there. Makes it easy to make both small and large pikes without any loss of development support tools for modules presently enjoying such privileges.