No doubt there are some things to be sorted out here. It might be possible with a G600 and KFC 300 to pass the system requirements. And it may turn out that a flight check with an AI on board certifying the results with an annotation in the log book may be all that is required.
I think the FAA whole point in this rule change is that ADS-B out gives ATC a very accurate altitude and position readout maybe eliminating expensive altimeters and RVSM certs. Otherwise, what is the real point here. The intent seems to be that ADS-B out aircraft provide ATC with sufficient, accurate, and confident altitude information that allows aircraft to operate in RVSM altitudes with the requisite level of safety. Before that, the only way they had confidence that you could operate in those altitudes was to require expensive equipment, go through testing, and apply for the certification documenting all that you did. Now they seem to be saying if your current system can fly to these standards and you have ADS-B, you can fly above FL280.
Time will tell but lets hope it gets easier rather than just eliminating paperwork. I appreciate all of the comments!