So after about a year trying to order a counter-clipping jig from a guy in the US, I decided to make my own. It was surprisingly easy - a redundant CD case, a hacksaw, some fine-grade sandpaper, and about 15 minutes careful work were all that was needed. The critical bit was filing away the corner until the amount of counter-corner showing was just right.
It certainly makes the clipping job quicker and far more accurate. As you can see I've been going through my ASL Starter Kit counters re-clipping them for absolutely uniform corners.
But the question remains - why would any sane person want to do this?