I must have misread http://ardupilot.org/rover/docs/common-optical-flow-sensors-landingpage.html not seeing the part I didn’t want to see (that Rover is not among the supported platforms). No correction needed. I realize there is a lot of overlap in platform documentation. I also saw https://github.com/ArduPilot/ardupilot/issues/7420 and that encouraged me in this direction.
I was looking for location capability with finer resolution than GPS and not relying on wheel turns. Computer mice are pretty close to the surface and they seem to track nicely, so it’s not obvious to me why a rover couldn’t have a flow device mounted close to the ground. I’ve seen at least one video of a rover using optical flow. They had a light shining on the ground to overcome ambient lighting variations.