ComputerDoc said, "Not that I'd want to be flying in a legal gray zone.... But as long as the ADS out was working and showing you up properly to suitably instrumented traffic and ATC (something you presumably could determine as true or not in the next 6 years) how would ATC know or care it wasn't TSO?"
MikeD said, "However with GPS, and the rest of modern communication and navigation equipment all computer based, they all could, and probably do analyze and "certify" available signals as accurate before they display them. Which brings us to the question. Why would/should GPS's, ADSB, etc need to be permanently mounted, and FAA certified in order to be used?"
RE: ComputerDoc, ATC would not necessarily know whether your system was TSO'd or not, any more than they'd know whether you had a functioning artificial horizon or working position lights at night or were filing IFR flight plans without satisfying currency requirements. Generally, you can get away with breaking all sorts of FAA regs until something goes wrong and the FAA starts getting in your business and/or your insurance company starts looking for reasons to deny your claim.
Re: MikeD, there are better places to debate the merits or lack thereof of the FAA's methods of regulating general aviation, but to provide one answer your question, when a system is powered by a potentially-ginchy cigarette lighter adapter, using antennas that are draped across the cabin and suction-cupped to windows that may or may not have a clear view of the ground or sky, then there are plenty of ways for the system to not perform well. TSO'd equpiment installed by someone who has to sign his name to it in your aircraft records is one method the FAA tries to raise the bar on equipment performance. If primary radars are going to be retired in favor of "self-reporting" by individual aircraft, then forcing aircraft installations to have a high-reliability GPS system (including a permanently-mounted antenna with a clear view of the sky) and a high-reliability reporting method (including a permanently-mounted transmitting antenna with a clear view of the ground in front, behind, and to both sides of the aircraft) all powered by a solidly-wired power feed doesn't seem too unreasonable.
Might not be a big deal if your portable system fritzes out and stops receiving weather data or stops getting traffic info. Might be a bigger deal if ATC is tracking you on an IFR flight plan and all of a sudden you disappear off their screens because your antenna fell off the plexiglas or the cigarette lighter adapter jiggled loose.