I recall a friend doing something similar with mobs for Quake 1. The game used its own dialect of C, iirc. And it was possible to decompile the relevant files. So after some time he started modifying mods.

That Quake 2 and later required Visual-C++ and such kinda took the spark out of the creative mods, as the entry price became too high.

For instance Quake 1 had a "flight sim" of sorts that was basically done by one guy using Quake-C and some simple, freeware, modeling tools.

There was an attempt at making a successor for it in Quake 2, but even with a larger team etc it basically ran out of steam thanks to the complexities involved etc.