I like that cart.... Perhaps you could modify it with a swivelly third wheel in front like shopping carts have.... those can be a pain too though because they sometimes go every which way and cause problems.
What I would want ideally is a solution that was built right into the case and allowed it to open quickly and easily.
Thinking about the neotech case I had in high school, that thing was huge, heavy, and rolled extremely well probably because of a combination of the width of the wheels (3-4 in) and the placement in relation to the hinge of the case. I think the up and down orientation of cases limits the effectiveness of wheels because they are completely limited in width and diameter due to the fact that they must accomodate a hinge in between the wheels and be semi low profile. However, if you orient the case so the tuba rolls on its side, the problem of how to get through doors arises....
Balance, as it has been said, is a huge problem in relation to wheels as well. All tuba cases I have had where the tuba rolls traditionally are tippy. So why not add a wheel or two in the middle to opposite end of the case to stabalize things? Those wheels could possibly be smaller than the primary wheels because of their function of stabalization.
perhaps I have found my calling..... making custom instrument cases!! Anybody want one?
J.B. Adams Custom Cases..... sounds professional.