I'm sure it COULD be done with some welding and fabrication. It is not a dealer option as I'm sure your aware of. Plus I don't belive it has enough power to make use of a 72" mower. I had a BX22 and the 60" was fine. I have a rear finish mower (72") on my new tractor but I also have more power/weight.
I dont' think that you would want to. When I had my BX2200 I had the five foot deck. Several times in heavy grass I had to slow down to keep from bogging down the tractor. If you go to a six foot, don't let the grass get too high.
That's odd because I often cut grass 8" - 10" high with no problem whatsoever. Sometimes the chute gets plugged but the machine never slows or labors.(with the 60" deck as well). Also I doubt you'd ever get it to work. The model specific decks just miss the rear tires. A larger deck will have longer blades requiring a wider deck, therfore it would not fit.
You must have been running at low RPMs or something. I cut thick wet grass all the time, sometimes its almost up to the hood and still runs just fine...the chute will plug if grass is tall on the deflector side, but the tractor never bogs.