Hello David,
The necessary changes are not yet completed because they require more work. That is why I did not yet reply to you.
The SASL package is updated, and it supports XOAUTH2 authentication mechanism. That allows the SASL package to authenticate using an OAuth 2 access token. This is already tested in my development environment.
What remains to be done is adding a script to allow you to get the OAuth 2 token to use with the POP3 client and SASL classes.
You can use one of the scripts to log in with Google from this OAuth client package and change them to get the token.
phpclasses.org/package/7700-PHP-Aut
...
Or, if you can wait, I can send a commented script so you can adapt it to your application.
Can you wait maybe a few more days, or do you prefer to use a script for the OAuth class?