If I am understanding your post correctly, this is essentially what people are talking about when they say a bike's ability to accept a large cog of a certain size has as much to do with the drop outs, as with the current RD's capacity? Because on this particular frame, the little nub on the drop out is very very small indeed - so I think you are 100% on target - it seems to engage the screw when it's in a few turns, but once you get past 1/3 or certainly half way, it slips off the edge.

I do have a NOS Ultegra long cage RD I could try installing, but am I correct in saying, with that tiny nub on my drop outs (short of the building up of more material you suggested) it likely won't make a difference? I.e., this frame was just designed to to use a small straight-block cassette with a racing RD?