You could always try to find out if the author is still actively rewriting the damn thing. Either by PM or by making suggestions about things that could be rewritten.

Other than that, I guess it is just as much fair game as anything else. I've seen a few fics I've PPCed rewritten (mostly after I tackled them) and those rewrites still left plenty to charge the Sue with.

I'd say it's fine; if it was last updated in '04, it's not likely to have the rewrite posted. Of course, I'm not a Permission Giver.