Ciao Rob,
I've did that in 2016, I don't remember nowadays the exact code.
Besides the patched file was "includes.php", close to the end, where the send button is generated.
I have created a custom field in CB with the max number of messages allowed; this field is changed from CB Subscriptions plans, after payment. Then, in that file, I've retrieved the value of that field for the current user and I've managed to print or not the send button in uddeIM.
That's everything what I have in mind at the moment. It would be nice to create a plugin or a "file patcher" for that purpose, but the requests are not so many and the creation is not so easy without the modification of that file (I don't like to modify core files, but I like to create overrides).
Hope that could help you, somehow.
Kind regards,
Gregorio