I went ahead and took a stab at this since I was already in the physics code, and it wasn't too difficult to add. Basically, I just made it so if you're pressing crouch and moving faster than a certain speed, the friction is significantly lower.
The most difficult part was fixing the extreme wobble (happens currently if you crouch-slide on ice really fast). I think that was a necessary overall change, so even if crouch sliding doesn't get used, that change is good.
It's a bit weird on hills and steps. I disabled stepping (well, capped it at 5 units, so you can still make it over tiny bumps) while crouch sliding so you don't skid up stairs at 100mph -- that just looks weird. There's not much I can do about ramps. I did fix a bug where it was possible to crouch while in the air, though, so you can't fling yourself across the map in the crouched position. I allow the player to stay crouched briefly when moving through the air slowly, so crouching down stairs and slopes doesn't keep bouncing you up and down, but past a certain velocity he stands back up.
I fixed the issue I was concerned about (using crouch to slow down) by only crouch sliding if you're pressing the movement keys in the general direction of the slide. If you stop or hold back, it uses the old friction.
It doesn't change a whole lot -- I don't think it's going to make or break anything. It does feel a little smoother when you land crouched and don't come to an abrupt stop. I'll try to post a demo later. I currently have it disabled by default (sv_crouchslide is the cvar name right now).
