Thanks for the offer and the feedback. 

I do have one other concern, and that is API stability.  If I do decide to go ahead with boost.extension and it is eventually accepted into boost proper then upgrades of boost versions means potential major refactoring of all components that use plugins.  Having gone through this pain in the past with other boost libraries it is a high level concern.  While an API change is expected now and then, having to rewrite too much code at once isn't desirable!  Any advice on how to protect against this?