Interesting. Post a link? That will be good test for that product. Fair amount of weight and vibration. Curious how that holds over time. Reason for not using U-bolts?
I think your reasoning for not wanting to compromise the ROPS by welding or drilling is valid. However hanging tool boxes, valves or other equipment from the ROPS can comprise too. I do it too.
Maybe combining the brackets for the tool box and valve mounting could add structural strength? Don’t see anything bad in what you are doing, just the opposite. Admirable problem solving to increase the function of the tractor.