Is it possible for WHMCS to use a custom O365 endpoint?

[RealJabberwocky]

Last year Microsoft decided to move away from generic SMTP authentication and start demanding the use of OAuth2.  WHMCS works great with the OAuth on the commercial O365 cloud, creating an enterprise app, app registration, and getting a token from the O365 cloud (office365.com cloud).  However, we need to use a different O365 cloud (office365.us cloud) and WHMCS doesn't seem to have the ability to send emails from the billing system using OAuth2 and anything but the standard commercial O365 cloud.  Is there anyone out there that has successfully gotten WHMCS to use a custom cloud for emails?  Somewhere buried deep within the PHP code to force WHMCS to authenticate with a different cloud?

The only other option we have to move away from WHMCS or come up with a kludge to send emails to a commercial cloud tenant, forward them to the .us cloud tenant, and THEN forward those emails to the customers...all while looking as if they emails came from the .us tenant.  I've got a partial kludge working with mail rules where the commercial cloud forwards to the .us cloud and changes the mail header "Reply-To" but the message are then being flagged as spam because the sender and reply-to do not match.

Any thoughts?  Ideas?