Well, I think that for plugins you will have to provide some sort of even little bit more complex interface (and more narrow in the same time) than simple dynamic linking.
Personally, if I would be about to provide a way how to write plugins for my apps, I would try for possibility to write them using some other GUI libraries...
And when writing U++ plugin for U++ app, I would rather kept U++ library as separate copy in the .dll / .so to avoid problems. (BTW, at the moment, there is quite extensive experience with writing U++ plugins for non-U++ apps).
In any case, this is certainly an interesting topic to evolve.