Thanks for that thread, Tony. It didn't turn up in any of my searches (probably because you used terms like "screen shades" where I tried searching for "greyscales").

I've put these values in my palette:
{ 0, 91, 115, 255 }

Colour-reduction's not my problem - the Gimp can reduce to that palette using fixed dithering, Floyd-Steinberg dithering (recommended - with the low-bleed option - for photographs), or no dithering (good for cartoony images).

The code's looking quite good, though I've now moved to Gimp 1.2. I'll post it when I resolve some issues with layer composition.