This is extremely interesting, LR. I have seen photos of Rhodesian Canberras with a white-painted nav hatch, to which I think might have been one of these frangible hatches. Not sure however if this was the case, but will check with Mike Hamence on it. Again, as has been pointed out several times on here, this the beauty of the Can as such a long-serving old girl - no two are the same, and even if this mod was introduced as Flight points out in 1956, we don't know yet how standard this mod became. At least a lot of photos point to the hatch continuing to be blown clear off for the SOP reasons previously mentioned.
Blasting through a canopy mind you must be one hell of an experience...not necessarily one I would relish, but then again, the alternative would make you not think twice about it.