I saw a hula hoop being conveyed on an Easyjet flight. The cabin crew had the hoop placed behind the last row of seats at the rear of the plane.
I was being nosy and asked the guy with it about how he was coping with travelling with it, and he told me the biggest problem was security, as the hoop was too big for the X-ray machine. The security people hand scanned using a portable device, and then swabbed the whole thing. It took a little extra time, so you might want to check in a little early.
I agree with mark about being upfront and getting airline staff involved in the resolution to the issue, but having a potential idea (ie behind the rear seat) may help you on the way.
I would also caution about checking or gate checking, as a hula hoop could get crushed or bent by having luggage placed on top. I would also worry about the ability of automated luggage systems / carousels to handle the shape.
If checking is your only option I wold suggest a sturdy carton or case if possible