For anyone checking out these possibilities, this could be done by setting up a lead action that fires off a webhook to Flow XO which then uses the built in FLG integration to update the lead's contact preferences.
The lead action would be triggered on the event 'Text Message Replied' with a condition that the text message body contains 'stop' (or any other words you want to trigger this on).
There is a guide here:
We will still review the above idea for consideration with future updates so please continue to leave any feedback and suggestions.
For any help using this method please contact email@example.com