I think I might cobble something together to separate the alpha channel out as a separate monochrome bitmap

This works fine: I can load the RGB bitmap into Paintshop Pro, and then load the alpha channel separately (I wrote something that exports it as a 24bpp grayscale bitmap for simplicity).

I can then edit the bitmap, although I have to edit the RGB layer and the alpha layer separately (e.g. when moving tiles) which is a pain, but I can live with it.

Unfortunately, I can't find any way to get PSP to save the damn alpha layer as a monochrome .BMP file, which means that there's not much point in me writing the rest (24+A -> 32) of the utility.

Bugger.

Any more suggestions?

I can't even find a way to get the Gimp to load the second bitmap as an alpha channel.