A few years ago I sat on a CAA/industry committee (now annoyingly abolished, since it did a lot of good for a while) which amongst other things was looking at IFR use of pemit aircraft - of particular interest being warbirds and PFA/LAA permit aircraft.
It all looked quite viable, and the CAA weren't philosophically against it. The basic aircraft requirements anticipated were:
(1) Primary flight instruments to the same minimum level and standard as would be required on a CofA aircraft allowed to fly IFR
(2) A certified engine
(3) Handling qualities of the aircraft assessed as to the same standard as would have been required for new CofA light aeroplane certified for IFR use.
(1) and (2) were pretty uncontentious, (3) would certainly have excluded some PFA types, but by no means all (not having flown an RV7, I have no opinion on that).
Unfortunately this all died for a variety of reasons that mostly had nothing to do with the commonsense of these ideas, which were fairly apparent to both CAA and PFA. In an ideal world it would certainly have all happened by now.
One thing worth mentioning, whilst you can't currently fly IFR in a permit aircraft, holding a current IMCR or IR, does substantially reduce your personal VFR minima, so allowing you to actually do quite a lot in a permit aircraft.
G