Well, the rename it to a .bmp, convert it to greyscale, and store it as a .jpeg approach worked okay for the one render I was doing, but that texture was relatively low res. I'm not sure there's a good way to get a one-to-one conversion. P4 .BUM maps use that embossed look, and P5 & 6 use the greyscale approach, which probably gives a more realistic look. I think that almost any approach is going to be more of an approximation that a direct translation. If I were doing it again on a higher res texture, I'd probably tinker a lot more with the contrast of the greyscale image before using it for a final render.