For AMG some exist but distribution is prohibited. You can build your own tho and keep it for yourself.
I'm sure somebody has made something. It would have to be an html parser I suppose (hardly ideal, and easy to break), because they don't have an API that I know of.
But I think this would be impossible since Last.fm is based on scrubbler (or something like this) and the only thing you can do with this is send what you are listening. So I think you have no way to send favourite.
Actually, they do have an API call that lets you favorite any track (whether currently listening to it or not). I whipped something up quick and dirty just to check it out.
I totally agree that it would be best if it was incorporated into the last.fm plugin. I looked around for the source code but couldn't find it anywhere, so assumed it was closed source. You could "catch up" by just queuing up all your 4 or 5 star songs and playing them.