I think there is a possibility to have a local client using Gmail, with a rule set to delete mails on Gmail as soon as they are synced locally. So in this case Gmail would be nothing more than a relay/proxy, while your mails would actually be offline.

I think this is the first subsonic android client that is not an eyesore. I wanted to use something like this for a long time but the clients were simply to ugly and aesthetics are important to me. I will definitely give it a try and maybe even submit some PR if I find something missing and have time to contribute. You have my star.