I can think of one really quickly.
Chair out of position in relation to the target. Also, target on opposite side of gunhand.
The limited mobility is going to decrease the manuverability of the student. This sounds like a real challenge. Also, look at having them shoot from out of the chair, on the ground. Often a chairbound person will find themselves in this position IRL.
You can also look at how they carry their reloads and have them do a lot of one handed drills because depending on the degree of disability, they may need to support themselves with one arm while they are on the ground, thus they would only have one hand/arm to use for firing.
I hope that helps. I'm not a trainer, but tried to think like a less manuverable person for a couple of minutes. Good luck and I'm interested to hear what you come up with.