Yes, this is quite possible - I'm pretty sure bmap_to_alpha uses the original bitmap data, not the written pixels. I'll check and if this is the case, I'll mention this more clear in the manual.

- bmap_to_alpha was a legacy function from A4 times when alpha channel textures weren't supported yet. You won't need it in A7 - just use 32 bit TGA already, and write 32 bit pixels.