Re: manual line locks??
They are still around. Check dune buggy or speed shops or search the internet for "steering brakes" or "cutting brakes."
There are several types around.
The third ratchet lever in the middle was a mechanical lever that pulled on the mechanical E-brake. They were made to fit the VW lever and ratchet.
You run the brake line from the master cylinder to the cutting brake, then individual lines to each wheel. If you wanted to add a mechanical line lock (1), simply put it in the line from the master to the cutting brake. You could mount it right beside the the cutting handles - easy.
The electric solenoid type would be much harder to use as a steering brake - like on a steep sand hill in a decent - push the button, step on the brake hard (may not be such a good thing at the moment to lock all 4 wheels), then let off, letting that wheel stay locked. When you no longer want it locked, find that button again!
Rock crawling it wouldn't be so bad, but it has several operations when you already have your hands (and mind) full.
Either way, you don't have much control over how much lock you get. You don't always want it to lock, sometimes just drag a little.
The lever type - just grab it, pull as hard as you need, then let go!
Of course the obvious - THE LOCKER MUST BE TURNED OFF!