If you're question using a ZTR on a slope/undulation - IMO the simple answer is you most probably shouldn't be using a ZTR there.......once youæ±*e approaching the maximum machine gradeabilty, any relatively & in most situtaions minor equipment failure especially a tyre deflation can lead to catastrophic results
IMO, quite why a ROPS is required on a ZTR is interesting, sure there may be legislative requirements & you can mandate all many of safety firments but this all seems a nonsense given if you're using a ZTR in a situation where a roll over presents any real danger you've exceed the design intent & capability of the ZTR, & more than likely the operator skills/instinct for self preservation .......all a bit like fitting a parachute to car in case you decide to drive it off a cliff........
The fitment of a ZTR ROPS IMO merely encourages false confidence/compounds "misuse" in terrian where it was not designed to go ....in these circumstances perhaps alternatives such chemical control, possibly using a 4wd CUT, or landscaping that hillside grass area is the better choice....
Consider also in reasonably flat or lightly undulating garden environment, a ZTR ROPS is potentially a far greater hazard/risk due to overhead "hand ups" & "back flip" than its original design intent of a supplementary safety device.
It's not that I am against ROPS or safety investments, in fact on our farms I would not have any tractor without ROPS or indeed any self propelled plant without flashing beacons, reverse quackers, seat belts ..etc (& have dismissed farm hands for not embracing safe practices such as mandatory use of seat belts irrespective of the terrain).
I'm sure others will have different perspectives .........