I think that BEagle sums up my thoughts on the subject, that the powers that be knew, but did not do anything about it. I cannot believe the comment by Freeman Dyson that ORS knew nothing of Schräge Musik. to me it just sounds as if he is trying to justify his own role in the bombing war.
Well it would appear that at least one bloke at B.C. thought he knew what was going on, and his frustration at not being able to persuade others and thus influence the evolution of bomber defence is pretty clear in this memo, don't you think?
Consider that the downward firing "scare gun", downward vision blisters (referenced by longer ron above) and the Rose rear turret combining heavier armament and - importantly - a much improved downward vision for the gunner, were all Bomber Command initiatives aimed at thwarting attacks from below. I think history has got this subject wrong.
p.s. The Havoc system was devised by Nash and Thompson (the turret people) and was rather more complicated than a S-M installation in that control of the guns was in the hands of an observer in the nose compartment. By the time it had been trialed, Fighter Command opinion was that the Beaufighter (soon to be followed by the Mosquito) had more than ample forward firing armament to tackle German bombers.
It didn't stop further trials with Beaufighter and Mosquito having four gun turrets installed along the lines of the Defiant, which would have allowed attacks from the beam - or from beneath...