I've written a tool to do this, committed in r342: /tools/monochrome-btg
Having written this tool with the express intention of using an all-black backdrop I discovered something about Homeworld's background rendering. Not only does it draw the .btg backdrop but it also appears to overlay a random number of scattered grey "stars" to help break up the background and prevent empty regions of space. This is exceedingly annoying as it means you can't have a completely blank background to take screenshots against!
If anyone knows where this gets done then please let me know, otherwise I'll take a look tomorrow.