Another little update post. Annoyingly, as soon as I got a nice working solution, scrobbling to libre.fm via Scroblr.net seems to have stopped working. I'm not sure why. I contacted the guy who runs it, but as of right now it is still not scrobbling to libre.fm for me.
So rather than go back to producing two separate log files via my post-fm modification I've tweaked the Libre.fm libreimport.py file to work with the scrobble.log format. So I am now back scrobbling once again.
(As an aside, I'm not entirely sold on the usefulness of Libre.fm. I was hoping it would recommend me free artists in a similar vein to how Last.fm provides recommendations in general, but it doesn't seem to do this. Although weirdly does suggest free artists my neighbours might like - perhaps it is because I haven't listened to any free artists yet?)