Just thought of this. Whenever I am working with someone in a power chair, I visually check the status of their control box power switch to confirm that the controls are deactivated. I would advise that everyone working the line be made aware of this switch, its function and location. For safety reasons, I would further advise that the shooter turn the power off while they are shooting. If they forget and leave it on, I would discretely mention it to them, but you should never touch the controls on someones wheelchair yourself. The power switch is usually a toggle located just below the joystick. When the power is on, typically the LED's on the control panel will be lit.
(http://i1120.photobucket.com/albums/l481/Unbridled_Liberty/Appleseed/ivcpsfsn8.jpg)
If you are not familiar with the power switch on their chair, just ask them privately to show and explain their controls to you, and take the opportunity to explain that, for safety reasons, you would like them to turn it off before they start shooting.