I finally found the safety guide on the site, and as long as you can fit in in the car and it isn't harm to others, they are allowed.
That quote was taken from the Worlds of Fun website and a similar one can be found on the Cedar Point site, I imagine there will be chain wide consistency this year, unfortunately no particular information is available on Kings Islands website yet. I think you'd be fine with the case so long as its not a full arm cast, but always ask your doctor first, you don't want to end up being in the cast any longer than you have to.
It only goes halfway up my forearm, and its not that big, so it should be fine