It's been a while but the most I remember seeing was someone made a tweak that made a few of the hidden items available. There's this forklift weapon that was never used cause the physics of the game don't allow it to work. And they never coded a way for it to work or something. There's a couple other components that are hidden I think but none of them really work or are worth trying to get ingame. I don't remember the tweak or if it's still around somewhere, but it's really not all that worth it tbh. I think the most I ever saw was a load of skins that are probably lost to the depths of time.
I want to say that was AW from AceUplink that tweaked the hidden items (Forklift, Wood armor, etc). But none of them work because there is no code for them, only menu graphics. The forklift is the only one with an in-game render and attachment points, but since the game has basically no physics the forklift doesn't do anything.