100 feet of buffer is inadequate. How the hell do you measure your AGL when you're flying? You either use a radar altimeter ($25K installed on an airplane worth $20K) or you use the baro altimeter, which has an acceptable calibration error, plus the local altimeter setting (atmospheric pressure) which has an error band, and there's error because you're not right over the reporting station.
Well, if you had a good GPS receiver and sufficiently detailed topographic maps on board you could also guesstimate AGL that way--but I agree that it's still a dubious and non-robust approach. And your radar altimeter doesn't have to run $25K if it only needs to work up to a few hundred feet and only be "hobbyist" or "drone" rated.
But really, forget measurement--that's probably not even the biggest problem. I suspect that it would be very technically challenging for these craft to physically maintain their permitted altitude. A good gust, an up- or down-draft, and your plus-or-minus 100 feet goes by in no time.