Here is a BIG extractor for HW1 Classic. Most probably works, although obviously I haven’t gone through every last file and done a byte for byte comparison with matches in HW1. Written in VB net.
Fairly simple - open a big file from the menu, and it’ll load. Then select extract and it’ll…
The idea is to allow full extraction of HW1 classic BIG files, and then allow people to overwrite with their mods, then use the steam archiving big creation tools to package a new homeworld.big with your altered files in it.