You can definitely hook up a payload with a UART interface to your OpenROV, but its going to take a little bit of work on your part.The first thing you should do is grab a schematic for the controller board of your OpenROV. You can find them on our Github site here.
There are six auxiliary wires that come out of the OpenROV housing that can be used for whatever you want. Note that on the OpenROV 2.8, five of those wires are pre-wired for lights and an external servo, so to use them for a UART you will have to break the existing connection to free up those wires. Assuming you have a model 2.8, I would re-purpose the external servo wires (TP23-TP25) into a UART connection. If you look at Sheet 2 of the schematic, I would remove R70 and R71, freeing up auxiliary wires TP23 and TP24 for the transmit and receive lines. TP25 can be left as a ground signal.
Then on your controller board you’re going to have to run wires from the test pads TP23 and TP24 over to a free UART port on the microcontroller. The microcontroller on the controller board is a ATMega 2560, the same as is used on the Arduino Mega. If you study the free microcontroller I/O that is brought to the header sockets (Sheet 2 again), and cross them against the I/O of an Arduino Mega, you will see that there are a number of UARTs free for the end-user. Note that the AtMega serial ports run at a 5V level. If your device is 3.3V or true RS-232 (12V I think…) then you will have level-shift the signals appropriately.
This leaves writing the software to get your telemetry topside. If you’re using software 30.0.3, Charles Cross wrote this post on how to develop software for your unit.
Hope this helps. Let us know how things progress.