TheOddOne, that's correct as far as it goes ... but you're still completely ignoring the other legal requirement, which is compliance with the Kinds of Operations Equipment List contained within the POH.
In my aeroplane, flight under IFR (inside or outside controlled airspace and in VMC or IMC) requires the following flight instruments to be fitted and operable: ASI; Altimeter; Attitude Indicator; Directional Gyro; Magnetic Compass; OAT Indicator; Clock; Slip-Skid Indicator; and Rate-of-Turn Indicator.
Since this is a requirement contained within the Limitations Section of the POH, it is a legal necessity for validity of the CofA.
I have several friends with aircraft with no electrical systems who fly non-radio who sometimes fly in accordance with IFR VMC OCAS below 3000'.
I rather suspect this is far removed from what the original poster had in mind with their query! In any event, your friends may well be doing so illegally if they're G-Reg! Schedule 5 of the ANO, paragraph 2(5)(e) requires radiocoms equipment to be carried for ANY G-Reg IFR flight, wherever they may be.