I don't know about git-send-mail, but mainline mutt has oauth2 support for imap4, pop3, and smtp. I haven't used it but it looks like it pushes the oauth2 toke

I don't know about git-send-mail, but mainline mutt has oauth2 support for imap4, pop3, and smtp. I haven't used it but it looks like it pushes the oauth2 token refresh off to an external script.