Did you look into Librelist[0]? They also offer rsync access to your archives, so making backups is trivial.
I guess you mostly want to fix your current problem but if it's of interest in the longer term, we're building an email-based tool to not only handle the mailing lists but also help turn their content into documentation - http://copyin.com.
We're going to be rolling this out with a number of Rails and Android open-source projects and if you'd be interested in speaking it would be great to include you guys. If would be of help please feel free to contact me directly: peter dot nixey at copyin dot com.
Obviously switching to a self-hosted mailman is an option too. But having no access whatsoever to the group, migration is kind of a pain since you have no way of even contacting everyone that was subscribed.