/ignoreBigfiles isn’t a usable parameter for Homeworld 1 classic. (It ends up showing the window with all allowable parameters.)
I was already using loose files by the way, just like last time.
It seems p2ambassador is right. The game looks for files in the exe folder, not data or root.
Unfortunately now the game refuses to start and gives me crash logs when I leave a modded file inside the exe folder. These crash logs are not generated in readable text (except for a few lines).
I hope a dev can take a look at this.
For the record, as a test I extracted singleplayer.script from Gearbox’ classic Homeworld.big file and modified the starting RU as a test.
Shouldn’t be something crash-worthy.
I also tried changing unit caps in unitlimitcaps.script. Same problem.
Edit: One of the crash logs: