I have not used a knee roller (yes they are allowed at Disney, as they are an assistive device), but I did spend 6 weeks in an aircast this past spring, part of which included traveling/flying for work.
Some thoughts based on that experience:
When walking (I was allowed to weight bear) I REALLY wished I had an
ECV; and at the airports I wished I had asked for wheelchair service as terminals are long walks with an aircast, even in small airports.
If you are flying:
Consider paying for a seat with more leg room. you will greatly appreciate this as it will let you move your legs around and adjust to find a more comfortable position.
The air compartments in the aircast will EXPAND A LOT as the airplane goes up in altitude. I did not thing of this on the first flight until as the plane was climbing my leg suddenly hurt A LOT. Fortunately, I right away realized what was wrong and deflated the air chambers (note: the mad scramble to do this was very awkward while seatbelted in a climbing plane...). I had to deflate at least two more times before we got to altitude. For the subsequent flights, I deflated BEFORE we took off once I was safely seated. I then only had to deflate once or twice as the plane rose, and I would just do it periodically so it was done before the pain started

Once at altitude, I inflated the critter back to the properly level and was fine. As we came down, the sair chamber shrank so once we were back on the ground I had to inflate them to the proper level again.
Depending on the style of air cast, you may need to check with your doctor or the place that fitted you re: the best strategy to use/how much to deflate by and such and what impact that is going to have for you re: your injury. e.g. can you fully deflate it or do you need to maintain some pressure and if so how much, etc.
Finally, my aircast came with one liner sock. I wish it had had a second so I could alternate wearing and washing. Mine got sweaty and stinky and had to be washed each day. Pain in the butt when traveling, and I was glad I was allowed to not wear the cast at night after the first week -- would have been quite the challenge if I needed to be in it 24/7.
SW