Rover doesn't currently support the optical flow sensor. It certainly could though, it's just a matter of adding a few includes into Rover.
Most rovers are too low to be able to use an optical flow sensor well I think... but we have a change coming that will allow using an upward facing optical flow sensor (or actually it can face in any direction).
All add it to the to-do list. https://github.com/ArduPilot/ardupilot/issues/7420