Did you end up implementing it in Thunderbird?
We're looking to do that at my company and I'm currently implementing OAUTHBEARER in cyrus-sasl (which then could be used in cyrus-imapd & postfix)
I haven't implemented it myself, and as far as I'm aware, it hasn't been in the past few years. If it is going into cyrus-sasl, then I can probably put together containers for testing (which is on my work todo list anyways).