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Well, they make that item for "regular" trailers. So, using a dolly for a 5th wheel would work..... I would say. I would say it would be like buying a trailer Toad and modifying it to work w/ a 5th wheel.....
The only thing I see different between a 'bumper pull' vs a 5th wheel is the tong weight and height of the hitch.
I wouldn't mind seeing pics of that custom 5th wheel adapter built. My saying is, if it looks legit (painted nice and all), you shouldn't have an issue. It wouldn't help to have a picture of a commercially sold unit just in case someone says something..... it's one thing for LE to say something about an obvious 'custom built' unit, but another to say something about a factory built unit that is sold.....
Heck, I have a 'home made' tandem semi truck converter dolly. Was done using a sliding axle group from a ****** unit and someone built an adjustable tongue on it (nice looking one too). I got pulled over and well, they said nothing. It was "safe"... chains, new air cans, thick brakes and 5th wheel plate was A-okay.
Below are 'before' modifications pics.