I've found it doesn't slip in single-leg if you've got it on a softer, more spongy rope that gives in to bite a bit more, like the Yale 11.7's, Tachyon, etc. It's the stiffer lines like static kernmantles and smaller diameter ropes that it slips on. It slipped a little on 11.1 Tendril, but my 11.7 luna holds fast (though it drops/jerks inconsistently when you try to lower yourself in single-leg mode regardless of rope diameter, which is why the manual recommends a Munter hitch below the device, fulfilling the purpose of a rope wrench.)
Edit: I'm not sure the zillon would work well for that SRT application with a wrench. A few things would be prohibitive...for starters, there is one less friction link (or 'zig', so to speak) on the zillon than there is on the zigzag, which gives it a less "wide variety" of variable friction. Less minute adjustment, so to speak, and it's pretty awkward with a Munter. The other issue, is that because it's designed to have slack be pulled to the side the way we typically do for a lanyard, the pulley actually prevents the rope from being pulled straight down through the device. If you try to put the free end in a foot ascender beneath the zillon, it pulls it at a funny angle and just doesn't track well. Maybe it could be played with to work well, but the one-less-link alone would give me pause.
