A formal IFR flight has to be filed via IFPS (Eurocontrol), on routes approved by the computer in Brussels.
It would be fun to know whether IFPS check that a given ICAO aircraft type designator is capable of being IFR approved. It would be an easy check, and you would get nicked as soon as the flight plan was filed.
Anyway I would never even try it. Formally, ATC are not the police, but some of them know about planes, and in some countries (Germany?) ATC is a kind of a branch of the police. You would need only one ATCO along the route to pick up on it...
Flying IFR in a proper IFR type but without any licenses/IR etc is much more doable and reportedly many have done it. If you know the flying protocol, the lingo, do the radio professionally, you will get away with it 100% - until you get a random ramp check. Or perhaps until somebody who knows you at your base airfield reports you following an IFR departure.
In the UK, IFR in Class G, undetectable enroute but you would likely get picked up asking for an instrument approach in IMC. The only way to fly an IAP would be on a mayday.
Certainly, if I was flying a non-IFR type I would have an ILS receiver (as well as a full IFR GPS etc) in there because ILS is the ultimate lifesaver.