While waiting for the various bits of the new ESC modules to turn up, I've returned to working on the robot arm/grabber for the ROV. I've been tinkering with various control methods and finally settled on using an identical robot arm but with servos which provide position data rather than acting as servos. Then, in order to get this information into the ROV software, I've used a Teensy LC board to emulate a joystick. The result is a robot arm controller which looks like a joystick to my laptop, and looks like a GamePad to the web APIs.

I've begun the write up here: https://hackaday.io/project/11770-robot-arm-controller for anyone interested.