I am saying that my issuing of a C2 or C3 code can be questioned at the time or a later date, so if I see a fault which could be questioned as it if C3 or C2, then giving a C3 you have raised the issue, so can't be said you missed it, but it does not force anyone to correct it, so if found it was to manufacturers spec, it is unlikely one will be in trouble for saying some thing is potentially dangerous where what is considered as a higher authority has said this complies with type testing.
So returning to the blank, having a metal blank if a wire becomes detached inside the consumer unit it can make a metal blank live, so any metal blank would require crimping onto the metal box or separate earth wire, so in real terms seems unlikely a metal blank can be used unless it is impossible for any wire even if there is a fire inside the consumer unit to touch the blank, so metal not really an option unless plastic coated.
With a plastic blank, to get fingers behind them to remove seems unlikely, the only way to remove without key or tool is to push them in. I am sure when pushing a blank in there must be a limit to force used to test, if I stand on a consumer unit made of plastic the whole unit will likely break, so I would say the same applies banging it with an elbow, so looking for protection against fingers or other object not greater than 80mm in length and 12mm in diameter (accidental finger contact). I am sure there is a limit to the pressure one can be expected to push with a finger? And I suspect in most cases you could not exert enough pressure with a finger to push in the blank. Use a screwdriver and you using a tool.