Created some nice mod templates for Blue Shift and Opposing Force, based on Half-Life Updated. It took me a long time to figure out you could make mods for these games without having to copy all the base game files. The magic behind this is basically the "fallback_dir" property of liblist.gam.
in hammer++ straight up "compiling it". and by "it", haha, well. let's justr say my .vmf file
It was through “fallback_dir” that I created my mod ( moddb.com/mods/horrible-incident… ) for HLBS until I froze it due to personal reasons.
Indeed, that property is really useful. A precursor to source's file system.
Valve only used it for CSCZ as far as I know