Though there are not fixed jetways at Owen Roberts Airport but there are four
mobile boarding ramps, a bit similar idea to the trucks with stairwells, except these are ramps that can be moved around. There are three large ones for the big jets and one smaller one for the Saab that serves the Sister Islands.. Cayman Airways bought them and use them but were hoping to make them available to other airlines for which they provide ground service. Not sure to what extent any other airline makes use of them.
For ground transportation of someone in a power chair I suggest making inquiries with:
These were from some of my old notes. All but Max Taxi still show wheelchair accessible transport in
this official government tourist webpage from more recently. (pdf at link)
Cayman does get a good review from
John Sage at Accesible Caribbean Vacations. He notes the availability of wheelchair vans, though lack of rentals vans with this capability. Overall he gives Grand Cayman his highest rating of any Caribbean island for its accessibility. That said there is still room for improvement. (Specifically thinking of one telephone pole right in the middle of a narrow section of sidewalk on West Bay Rd which totally blocks access.
).