I'm a car seat technician (to give you some validity to my statements
) and am on the more active/hyper side of being safe than most!
When we went to WDW in December w/ dd (6yo) and ds (2yo) we took ds' car seat (Britax Boulevard) to use on the airplane (rear facing!) then placed it under the DME bus for the trips back and forth to WDW. For dd, we didn't bring anything (she was using a Britax Husky at home which isn't FAA approved for aircraft). Once we were on property, we just used WDW transportation and stored ds' car seat in the corner of our room in the "closet" area.
There is no way to use car seats on either DME or WDW transportation on property due to the lack of seat belts. Bus travel is different than car travel when it comes to safety. The DME buses have compartmentalization (ie the seat backs are high and are designed to protect you in a crash situtation) and the buses on property are similar to city transit buses which travel at slower speeds and on familiar routes.
Ds slept on my lap for the trip to WDW on DME and just sat there for the trip back to MCO. On property, he either sat on one of our laps or in his own seat depending on the crowd level on the bus. He did great!
I was fine doing this (and my almost 7yo is still sitting in a harnessed seat at home!) and would do it again in the same situtation.
If we were renting a car, I would have taken dd's Graco Turbobooster (back up seat) broken it down, placed the back portion in our checked luggage and taken the bottom "low back booster" portion on the plane as a carry on. Once we arrived at MCO I would put the pieces back together again. This way, if our luggage was lost or damaged, I would still have something for dd to use in the vehicle.
Does that help any?