I'm sorry, but I don't think Disney offers anything besides buses, boats and monorail for guests.
However, there are plenty of taxi and limo companies available if you should choose that option.
I can see where busing to MK, then monorailing/boating over to the Poly would be quite taxing for a wheelchair-bound guest.
Here's a possible alternative, although it may not be any less tiring: make EPCOT the park you visit on the day of your 'Ohana ADR that night. About 45 minutes or so before your ADR, make sure you are near the front of EPCOT, then exit EPCOT and hop on the monorail there. There are elevators in some of the monorail stations (not sure about EPCOT's) for wheelchair-bound guests to use. Otherwise, you have to deal with the ramps.
The EPCOT monorail will take you straight to the TTC. Hop off the monorail there at the TTC, and walk a 5-minute walk to the Poly next door. It's a very easy, and fairly flat walk, even when pushing someone in a wheelchair or stroller.
This route (assuming you are already in the front of EPCOT) is a little bit easier than the bus to MK, boat/monorail to Poly route for wheelchair-bound guests. But it assumes you will be able to walk around EPCOT, which I honestly don't know if you wish to do.
Another note: if you do end up going to MK first, the boats to Poly and GF are not wheelchair-accessible. Any wheelchairs, strollers, etc. must be folded up. The boats to WL and FW are wheelchair/stroller-accessible, in that you can roll them right onto those boats. Monorails, of course, are wheelchair-accessible.
Here's hoping the ailing member of your party has an extra magical visit!