Hey, I’m busy on core features at the moment so my feedback may be slow. If you can get one of the coders in the forum’s excited about the feature, that may speed things along.
In the meantime, the firmware has a p,v, and s value which are the target values in micro seconds for the motors. By changing those, you change what the motors will be set to. My guess is that you would probably want to place the code here: https://github.com/OpenROV/openrov-software-arduino/blob/master/OpenROV/Thrusters2X1.cpp#L199
Right before the actual values are assigned to the motors. You can then figure out some logic that notes that there is a vertical motor command (v), and then you can induce a counter yaw my adding a value to § and reducing the same value from (s). You will also want to make sure to remove that counter yaw when the vertical value zeros out. To do so you will probably need to add some variables to track that you have introduced the counter yaw so that you know to later remove it.