Compare Versions

In the integrated CVS operations, the "compare with..." function is very powerful and i really liked it. but why is it that its only limited on comparing sources from the CVS? i want to compare my copy with another copy located outside the CVS. is this possible in IDEA? if not, is there a plugin that does it? if not, how difficult is it to create a plugin to do this? (re-using the IDEA-CVS library for doing the comparisons and just adding a menu item invoking the dialog boxa).