I don't know the answer to this, but I do know that you void the 3DR warranty by attaching any camera besides a GoPro. FYI.
I have got a pretty good idea how to do it. Its just to bad 3DR couldnt have made the expansion bay connections easier to use.
Same process as with the PX4?
the issue is PWM control is a challenge. The main board had a 5v pin and a 5v ground you can solder the power connection to and the PWM control is part of the expansion bay. Just need to figure out how to connect it properly. Its to bad 3dr had to make the expansion of the solo so F'd up. Why they didnt just stick with what everyone uses is beyond me.