PNG format: 8-bit or 24?

[Chipp Walters]

I seem to remember going around this awhile back and thought someone might
remember if there is a solution.

If I create a grayscale (or any image) and it has fewer than 257 colors in
it (by default grayscale images do), is there a way I can force the export
image command to save it as 24-bit and not 8-bit? For some reason, the
export image uses 8-bit when exporting images with fewer than 257 colors,
and 24-bit for images greater than 256.

Anyone have a header rewrite or other hack to force the save in 24?

TIA,
Chipp