I think there are a couple of members over at the GB64 forums who started to update the GBA but stalled.
Surprising really when you look at the regular updates to GB64. The Amiga scene then and now was, is just as big as the C64.
Saying that the GB64 has a team I think who work on it, so maybe if a team work on the GBA and people take a letter each and update or something. Must be a lot of work for one person.
Here's the thread they got as far as updating A&B,
http://www.gb64.com/forum/viewtopic.php?t=4876
I don't mind joining in to help if someone wants to try and arrange something.