Why re-invent the wheel. Look to America where there are many "experimental" aeroplanes that are IFR approved. Look on Controller.com and I am sure you'll find many Vans aeroplanes "IFR".
Regarding professional fitting - well anyone can fit an IFR GPS, and as long as they do it to certain specifcations (i.e. antenna in a certain position etc.) then I can't see a problem with it.
All you require to fly IFR is a single VOR and AI really. I have flown PA28's with little else IFR.
One thing though, many people I know who have funky aeroplanes like the RV6 (which I really like) and an IMCr don't really pay too much attention to the rules and might fly it in IMC enroute anyway.