I think, the duty of flext would be just to provide that handle to the object's box (along with some means to control it) and let the user use whatever GUI generating interface he likes (if portable or not).
i don't think so, the principle is that the external sends messages to a graphic server which is embedded in the application's GUI, that's TCL/TK for PD, don't know of MAX's.
This graphic engine in return generates callbacks which are registered by the external, so you have to know what messages are delivered by MAX's graphic engine.
There's no way of modifying those messages as we ( except one of us ??? ) don't have access to MAX's sources.
Good luck !!!
Yves/