Well pondering this it occurred to me that since the texture coords were known to be useless why not just shift them to the centre of the existing yellow triangle without changing the texture at all.
So my next attempt did just that – so this is a new general10 shader version . Then it occurred to me that I could use the Y position (height) of the fin of the vertical triangle to move the texture coordinate out of the yellow near the top of the fin.
So here is a soon to be released shader version with no texture change at all. If you look closely the top of the fin is cut off and there is now a red line running along the flattened top. That’s as close as I can get to the DX9 version without getting silly and attempting two shader passes.