Operating "certified" ADSB equipment in an EAB is where things get tricky. While it is correct to say that that the FAA does not regulate how you install equipment in experimental aircraft, they do, or will regulate how ADSB out equipment is used in "ADSB required airspace" in the future. Currently there is no guidance that I am aware of that outlines a process that would prove the installation of "certified" ADSB equipment in an EAB would meet the system integrity level required of AC 20-165A.
You say it yourself, and yet you fell into the same trap Mac did...
FAA Advisory Circulars are not regulations!
The regulations are 91.225, 91.227 and the TSOs.
Keep in mind the system integrity level includes testing to ensure antennas are located so they provide 100% signal coverage during all phases of flight to ensure there is no loss of position reporting, position "ghosting" etc. this is why the FAA is requiring an STC for installation on certified aircraft. Once the first article STC has been accomplished and proven that the system installation and system performance integrity is acceptable, then if follow on STC installations are performed correctly as per the STC documentation you will be assured that the installation wil meet the SIL performance requirements.
I don't think THIS is correct, either. I can't say about why they require STCs on TC'd planes, but isn't that what they always do for TC'd aircraft?
Now, from the (non-regulatory) AC, about SIL:
(3)
Source Integrity Level (SIL). SIL is typically a static (unchanging) value and may be set at the time of installation if a single type of position source is integrated with the ADS-B system. SIL is based solely on the position source?s probability of exceeding the reported integrity value and should be set based on design data from the position source equipment manufacturer. Installations which derive SIL from GNSS position sources compliant with any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 which output Horizontal Protection Level (HPL) or Horizontal Integrity Level (HIL) should set the SIL = 3 because HPL and HIL are based on a probability of 1x10-7 per hour. Do not base NIC or SIL on Horizontal Uncertainty Level (HUL) information. If integrating with a non-compliant GPS, SIL must be set to 0.
(Emphasis added)
Having read all of the current FAA guidance I don't see how you could be legally operating "certified" ADSB equipment in an EAB while squawking anything other than SIL=0 since the installation is not FAA approved or accepted for use in the NAS.
Which FAR says this? Which ones says that the installation in an E-AB has to be approved by the FAA or "accepted for use"?
FYI Advisory Circulars are never "mandatory" nor are they "regulation" they are a FAA documents that provides guidance for complying with FAA regulations, same applies to FAA Orders and FAA Policy Letters.