Well it seems there was something problematic with my installation, because I updated to the latest SVN, exported the normal map from GIMP in BC5 format, and it works fine in game. I'm not sure what the problem was before — perhaps I was just testing with a game DLL which didn't have the latest changes.
BC4 definitely doesn't work but according to the description here, this is a 1-channel format so it wouldn't be expected to. I'm pretty sure I tested all of the GIMP formats the first time, but maybe I was just being dumb and tested BC4 multiple times but never tried BC5
@duzenko I've uploaded the images here if you want to confirm that the formats are correct. The diffuse is DXT1 and the normal is BC5, although in this particular case I would probably keep the normal map as PNG since this is less than 10% of the size of the DDS (because it's a smooth gradient normal map which compresses well with PNG).
@MirceaKitsune Feel free to download the linked archive and see if you can open the _local map. I'm 99% sure you will be able to because I can read and write it fine on Ubuntu using GIMP.