I certainly do not agree that because an aircraft is on an IFR flight plan, that it is assumed to be IMC at all times.
Even in Class A airspace in VMC conditions pilots have a responsibility to maintain a traffic watch. In Class E airspace, there could be VFR aircraft not even in communications with ATC.
I didn't mean to imply that the Concorde's suspension of operating rules was part of the type certificate per-se. I suspect there was some altitude, below which, the Concorde crew had to be able to maintain a traffic watch in VMC conditions. Otherwise, it was a form of Russian Roulette.