I would have to agree with Laurent, this is jsut far outside the scope of SFML, and far from cross-platform.
Why? I know i have no vote here, but still I don't really see any factible argument of why NOT:
Pros:
-Can copy/paste clipboard into/from the game from/to th e native OS in a cross platform way, useful for complex in-game or app GUIs
-It's a very simple feature, easy to mantain, great for those who write their own in-game UIs
Cons:
"far from cross platform" <- makes no sense to me
"far outside the scope of SFML" <- it has uses in games and apps programed in SFML, plus it's easy to do
"makes no sense since developers deploy their stand alone gui libraries" <- haven't seen the case yet