I agree that it would be really nice to have a plugin or facility to be able to fix playlists from within the Media Library as you describe.
But since there is no such plugin AFAIK, you might want to check out ListFix at
It's a java utility that will do exactly what you describe. Written by a forum member I think. I've even used it to fix broken Media Library playlists (you know, the ones with the funny names like plf1B5C.m3u8 under %appdata%\winamp\plugins\ml)