TAL 4/98 mentions two experimental pad detectors during the Toucan development, a fibre optic type at Warwick and a piezo-electric type at Nottingham. I also recall there being a trial of a tactile paving pad in Reading at some point (Castle St roundabout outside Police Station if I remember correctly).
I suspect these trials weren’t successful as I’ve not seen or heard anything about these since.

If the wheelchair has enough metal it might be detected by a magnetometer, you could try one of these in the footway as an experiment.
http://www.radixtraffic.co.uk/products/below-ground-vehicle-detector.html