Aikido from a chair
Years ago, while I was training with Ace Atkinson, Sensei, at UC Riverside, we had a paraplegic man join class. This provided a challenge for Atkinson, Sensei, but one which he rose to.
Mainly, he emphasized simple wrist techniques combined with drops. Since this man had use of arms, and since he used his arms to get around, once he latched onto you, you were not going anywhere anytime soon. He learned how to move and use his mobility in the wheelchair to his advantage.
In terms of practicing in a regular chair, there is a safety issue involved. Be aware of the possibility of back injury if the chair is knocked backward with you in it, since the back of the chair rests across your mid to upper spine.
Perhaps it is safer to first train in seiza then move to the chair.
Yours In Aiki,