Last year a contributer the the HTF posted a photo of a hitch receiver he had welded to the hose storage bumper. I believe he installed it to mount a bicycle carrier. The photo was an "after," shot of nice hole in the rear face of said bumper where the receiver had been attached. Again, if my memory is correct, they had hit a bump that caused the loaded bike rack to violently heave, tearing away the bumper face as the receiver, rack, and bikes went on a side trip of their own!
I looked for the thread but couldn't find it... however there are several threads from that period that dealt specifically with rear hitches/ piggy-back towing/ etc., that you may want to review.
The rear bumper on trailers (not just Hi-Lo's) is VERY light duty, and is actually meant to crush and absorb a portion of the force of a rear collision impact. Like a chain is only as strong as its weakest link, a hitch mounting system is only as strong as its weakest component. I think that reinforcing one of these storage bumpers is like building a house on a weak foundation... very high risk. Jack's idea of connecting a hitch mounting system onto the longitudinal main frame members makes the most sense... just don't underestimate the adverse effects of having much greater weight on the rear of the trailer. That extra weight (and I'm thinking also of the second trailer hitch weight) can put a whole lot of stress on the frame, axles, and tires, and may exceed the Hi-Lo's gross weight limit.
1967 17' Hi-Lo Bon Voyage (1967-1969)
1969 19' Hi-Lo Bon Voyage (1969-2011)
2004 27' Hi-Lo TowLite (2010-present)
2015 Chevy Tahoe LTZ