IFR/IMC flight with non-certified IFR equipment

[Marc Zeitlin]

so, question remains, does anyone have any experience on what owners have done to document compliance of a non certified GPS interface?

Owners have not done this because in order to prove that a non-TSO'd piece of equipment meets the requirements of a TSO, you'd have to go through the whole range of TSO testing. In which case, if you passed, just get the damn TSO. Since no experimental EFIS/GPS manufacturers (Dynon, Garmin, etc.) has done this, there must be a good reason - it's hard and a lot of work, and their equipment does NOT meet the TSO requirements (RAIM, etc.).

It's NOT legal to use non-TSO'd or non-TSO'd equivalent equipment for IFR, and no-one will tell you that a non-TSO'd piece of equipment meets the requirements of the TSO.

As far as flying RNAV approach in IMC, I don’t intend to, but would like the legal ability to file IFR with GPS as primary navigation and maybe shoot RNP approach in VMC if available.

No can do. Either install a TSO'd navigator (GPS-175, GNS-650, Avidyne IFD-440, etc.) or don't shoot GPS approaches or use GPS as the primary navigational means enroute.

Now, since it's legal to use a ham sandwich as the primary means of IFR en-route navigation (i.e., pilotage, dead reckoning, etc.) as long as you file your route assuming that primary means, you can use any GPS (or anything else) as your SECONDARY means of navigation. But this only applies en-route.