Barometric geopotential altitudes
To put it all together and conclude from what I've read on forums and found in references:
ADS-B definitely reports barometric altitudes that are most likely expressed in geopotential feet.
To convert these into orthometric:
1) Convert geopotential to geometric for all reported altitudes including the fixed TA and variable TRL.
2) Apply QNH only to altitudes above TA and TRL using the barometric and inverse (hypsometric) equations derived from ICAO Doc 7488/3 expressed in geometric heights.