Some local authorities in England do already use non-standard markings for advisory bays; Kirklees certainly do (or used to do).
Resident permit areas and advisory disabled bays throw up all sorts of grey areas and the view I take is don’t attempt it if you hope to enforce. Some argued that the “except in marked bays” clause on an entry sign would permit a blue badge holder to park in the advisory bay without a permit, and I agreed with that view. You could reasonably argue that the advisory bay was exempt; especially now a sign is not required for a disabled bay.
We really needed the streamlining of TROs to be carried forward with Signing the Way, but as that did not happen for various reasons I suspect local authorities are stuck with this dilemma – is compliance with the Equalities Act better than compliance with the TSRGD? Refusal of a bay on cost grounds alone wouldn’t wash with the former as far as I can tell.