How thick is your grass? What about a low-er-tech version ... stiff rods that hang down between the front tires and go down to the level you are cutting. The rod comes into contact with something solid and a sensor lights up inside the cab.
Just an idea.
Well, thick enough that I can’t see the rocks...
But that may be a good idea.
We just moved down permanently. In the past, I could only cut a couple times a season so the grass was knee-high and finding rocks was a problem every spring. If I can get the fields cut and maintained, it will be less of a problem, although they do seem to pop up out of the ground every year.
Another idea was some sort of scraper - almost like a cow catcher on a train. Maybe hinged with a riding wheel as we have pretty rolling terrain. Idea was any rocks low enough to hit the bushhog in back would hit the scraper in front and either make a noise or get pushed out of the way.