I can assure you there are many F100, Ba146, PC12 and at least one HS125 and Citation
There are 34 PC-12's in Australia. About half with the RFDS and half in private hands. Most have steam gauges (maybe only 8 have glass panels). As do early Citations.
My (imperfect) understanding is that the old steam gauge ones are like a GA aircraft. They will have a GA type avionics rack that will take a Garmin 430W 530W / GTN650 / GTN750 or even an old 480. Couple this with a Garmin 330ES transponder that you fit in place of the original flat pack transponder and its all done.
The newer glass panel aircraft are a completely different kettle of fish and clearly (from Dick's description) require type specific service bulletins. My guess would be (and its only a guess) is that any cabin class aircraft (turboprop or jet) that has glass panel displays will be in the same boat as Dick.
A number of companies are working on discrete ADS-B boxes that can be remotely mounted, but I don't think any are commercially available yet. There are also companies working on C146a GPS sources that will remotely mount specifically to give data to the transponder (so you don't need new GPS display units) but I think these maybe 2 - 3 years away.
And if our regulators didn't know all of this before they set the implementation timetable, they should be slapped.