This has already been done! You can send position to the ROV via NMEA on UDP port 27000. If the USBL software does not support it, there are some free programs that will transport the data from COM port to UDP network ports.
You can control roll and pitch with some different button functions. There is one to toggle a joystick between forward lateral/ and roll pitch trim control, and also buttons to increment the roll/pitch trim when pushed.