Still no joy.
Today I tried a new tactic.
Because I can do an export with a .tga bumpmap, I did this. This on it's own would crash the game. So I took a normalmap.dds and inserted it into the igb in the classic way, thinking the result would be an igb with a DDS normal/bumpmap. And it is, but it also crashes the game.
If they'd just given a little bit more information in their so-called "help" file (like what *&^%ing format to save the bumpmap in), or if there were a way to ask the devs, I wouldn't be wasting my life on this...