Have to say I have enjoyed this plugin. I have one question that may be an easy fix.
I changed the code as suggested to the avatar.php file, and it looked like the avatars were now working across the multisite. However, when you go into a group admin and upload a new avatar, you get a 400 server error.
Converting the code back gets it working again. but the avatars only show up when in that group, when in someone elses site, they are the generic unknown man from buddy press.
Minor issue, great plugin.