Pete when one talks about the onset of high speed buffet, we as you know are talking about a speed where air is starting to flow at supersonic rates over parts or a part of the wing. This occurs at Mmo in stedy undisturbed flight, not at a speed below Mmo, ie the bottom of the high speed maneuvor margin. The margins or if you prefer the envelope is small in this area of flight. I think you said earlier 0.3g is not much of a margin, If you look at wing loading in banked flight you are takling only a few degrees, however the margin is still there and I stand by my earlier comments.
TR3 if the system detects a compromise of the 0.3 protection buffet alert will indeed display in the scratch pad of the CDU, it will also display for other reasons, however this is an FMC Advisory Message. I am no test pilot either but I base my earlier statement on having flown aircraft which demonstrate quite nasty habits at high speed and some which even required stick pullers for cirtification.
Best Rgds