It's not perfect, but it's working :-) Currently mining a 33x33 chunk with this, and had VERY FEW ISSUES.

I spent a weeks time developing this third iteration, and I now have a fully functional, automatic gantry-based miner "WorldEater", that moves itself depending on a Pulse Repeater-signal.

It has been with Threshold Switches, as was mentioned in the Reddit post (I didn't know they could detect the "state/length" of Rope Pullies).

Right now it's mining 8*48, but could definitely also mine a whole chunk at a time (16x48), either by extending the gantries it runs off of, or by repeating the Redstone logic that is making it move.

Currently I'm working on a "brake pad"/"turning system", so it doesn't just go indefinitely in a straight line, but you could build some stations like you would with a vanilla world eater.

How I've gone about this so far is basically with chunk-loading (Openpac). I chunk-load the area I want to mine until, and when it reaches that destination, it stops (if I am not in the area).