Not at the moment that I'm aware of. C2 doesn't have any capabilities to handle this that Im aware of. With that said, you could always do something fancy with git. have a single repository with your common used code and then fork it for each new project. From there you could pull from the main branch everytime you have a change. Just an idea.
Just tried this.. made a directory common/ inside 'Event Sheet/'. At the .caproj file, referenced one (<event-sheet>common/Button.xml</event-sheet>), everything worked just fine. But when I saved the changes, C2 copied the Button.xml file to the 'Event Sheet' directory, and erased my reference line from the .caproj