Play: Can a coach be on the field in a wheel chair?
 
Rule: Yes. In a previous ruling it is accepted to have a coach in the coaching box in a wheelchair. The following guidelines are to be used:
a) The wheelchair not requires an assistant to operate it.
b) The wheelchair able to operate manually if the motor fails.
c) Any sharp protrusions are padded
 
It is important the individual in the chair be able to operate the chair manually because automotive chairs can fail and are usually hard to move out of the way.  

This ruling is consistent with ASA and Little League policy. You would need to check with your insurance carrier to determine if they require any further pre cautions. (Softball Canada Interpretation)