A few things.. if you don't get it to the level where it might starve the pump it would be bad, but short of that, usually when your dumping, your not driving.
If your cyl on the trlr is actually double acting with a vent in the "other end", you could make it double acting with a return hose and the cyl. would become a mini reservoir with the returning fluid going back to the tractor.
I did this on a trlr. with some qc couplers. I could switch back and forth from the tractor to the elec pump for use with the truck.