My first thought is you're putting forces on that hitch it was never designed for.
First thing is torque. Say you have a Class III hitch that's rated for 500# tongue weight / 5,000# trailer.
A crane with a 3' long jib, because of torque it's only going to pickup 133# before you're putting more then 500# of force on the hitch. Of course the hitch is built to go down the road, which means the trailer bounces up and down so I'd assume a 500# tongue weight puts more load on it from time to time, so maybe that's OK.
In addition to the force in the up-and-down direction from the lifting the load, because you're lifting from the top of the pole that supports the jib, there's going to be rotational forces in the front-and-back direction (just like we use a peavy to turn logs).
Like some of the guys on those links suggested, if you had an extender that transferred the force to the ground it would help take a lot of stress off the hitch.
Once in a while use you'll probably be OK. But I have to think if you lifted anything heavy regularly you're really risking damaging the hitch. And I'd have to think if you tow with the truck, having metal fatigue cause a hitch failure would be a very.bad.day.