To save an 8-bit BMP, just convert the image's color mode to indexed color first (you may have to flatten your image), and then save as BMP.
On the other hand, if you want to tweak the colours to get a better 8-bit image quality, you can use the Ctrl-Alt-Shift-S method first, save as GIF, and then reopen that GIF and save it as BMP.

atwl