[Alan Lacey] “If it’s not possible to do this within a single texture, how about mapping two materials to the object, one with the colour channel mapped to the correct postion and the second defining transparency? I tried this with with colour and bump bitmaps but couldn’t get anywhere.”
That’s the way to do it AFAIK.
Place different materials on the object in the desired order (won’t matter what order if you are mixing things like bumps but will matter with transparency involved) – farthest left is the bottom material – then check Mix Textures in the Texture Tag dialog. Check the first material you want to mix onto another. So if your farthest left is a base color/color map that would be usually be unchecked. If the next texture is a bump map you want to add that should have Mix Textures checked. If more are added it will go on mixing the new textures until the next Texture Tag that has Mix Textures checked. (simple version – two textures to mix? First is unchecked, second is checked)
Mixing textures is additive so watch what channels are turned in the various materials on so you don’t get unwanted mixing.
It used to be neccessary to have bump active in both textures if you wanted the materials to mix in a bump but somewhere along the line they made it easier and now mixing happens with only the Mix Textures box checked.