I'd think it would be easier to build a trike with a wheel chair style front to take a disabled person, rather than build a machine to connect a wheelchair to. In other words have the disabled person swap machines rather than attach a wheelchair to a contraption. Wheelchairs are generally built to take low loading as they move at walking pace so utilising one leaves a lot of strength to be added. You could effectively design the machine without having to compromise around the wheelchair and would end up with a lighter end product. Also if you make it so a wheelchair attatches you confine that disabled person to that specific chair. I can only see one advantage in making a wheelchair attatch and that is that the user would have the use of the chair at a destination. If that is wanted then a lot of compromises, weight and extra engineering is needed.