The concern is that you could blow the hose off the flush nozzle. On mine that happened and I had to drop the sealed underbelly then remove and replace the soaked insulation. Fortunately it was fresh water.
Of course a year or two later the same rinser with it's hose now securely clamped onto the inlet caused my black tank to rupture stem to stern. Yep I got to pull the belly again and remove the insulation again, only this time it was a much more foul job.
Unsolicited advice, leave the dump valve open while the rinser is running. :)