Carrying a dirt bike on the hitch of bumer pull TT?

That's an awful lot of weight/leverage to add to a trailer bumper. Most aren't designed to carry much more than a sewer hose and a spare tire. Weakest point is often the welds.

I wouldn't automatically assume that since the frame's heavy duty the same is true of the bumper. You might inquire of the mfr. as to limits there, or at least get the opinion of an experienced welder.