How common is arousal during AB play?

It's already been pretty covered. Try not to sweat it too much and communicate it with your caretaker. If you already have this intimate and trusting of a relationship hopefully bringing something like this up shouldn't cause any problems (even if it may be a little awkward at first).

Not sure if you're okay with this or if it's different for someone who's transsexual, but you could also try to handle your excitement beforehand. While the arousal response can't be permanently suppressed or satisfied for an indefinite amount of time, taking care of business, so to speak, should at least temporarily suppress or reduce any sexual arousal so not only will it reduce the chances of getting aroused, but it will also feel less sexually arousing.

Also, arousal during AB play I'd imagine to be more common than uncommon. Honestly, much lesser things can often cause arousal, lol.

I totally understand your concerns. When I'm changing myself I never have trouble as far as getting aroused goes. Once my significant other goes to change me, instant arousal. I typically am wearing a baby diaper as a stuffer, covered by a goodnite, covered by an adult cloth diaper. With this combination arousal does NOT work with fitting everything which causes some problems with changes by her. True ABDL diapers are much large and may accommodate this better, but I honestly don't have a ton of experience with it.

Just enjoy your experience and don't worry about things that happen that you can't control. Just enjoy the experience and have fun with it!