It's a nice idea, but it seems like a bit of work though, since we already do these kinds of thing at our own league websites/forums (generally speaking from a league manager's standpoint).
Not to mention, you would either have to have a common format for submitting (which may not coincide with league manager's native formats), or have everyone do it manually, which would not be appealing to me.
From a non-league manager standpoint, it would be very cool to be able to look up a single person's results from multiple leagues.