I would think it was not all that safe. As long as the tow vehicle could handle the hitch weight, I would imagine it would not be a problem though, but you are totally negating one of the best reasons to get a 5th wheel in the first place, the stability it offers with the weight right over the rear axle.
I've seen a bumper-pull conversion hanging on the bumper of a 4 wheel drive pick-up...
(in fact, it was the first Scamp
5th wheel I had seen) - buzzing right along about 70 mph.... My thoughts were "what is that? A fruit-picker conversion?" Sorta reminded me of the 53 Chev station wagon I had seen in my younger days that was made into a camp trailer being pulled by some migrant workers in Southern Idaho........