I'm thinking about LastFM plugin right now.
The problem is the mocp calls OnSongChange script only when new song is started and passes information about NEW song.
It's good to show nowplaying, but not good for scrobbling, because if you switch to the next song in few seconds after another song has started to play, both will be scrobbled.

Right now I'm trying to add option like OnSongFinished, which will trigger the script before switching to next song or stopping playback, but my skills in C is not right now, so any help is appreciated.

Also, you can check lastfmsubmitd, which implements offline storage and other stuff, but needs some love.