I got a knob!
Please excuse the "currently held on with tape and hanging off slightly as a result" thing in the positioning mockup.
It's exactly what I want, right down to size. Gonna fiddle with positioning a bit more, and gotta, well, get the rest of the desoldering done, wire in the attenuator, and then also take the bezel off and frickin' drill the thing. Gonna try doing it by hand, I think. Two holes, straight down, What's The Worst That Could Happen™? Well, three holes I guess - there's a nubbin on the attentuator to prevent rotation, which I should probably use instead of filing it off or whatever.
Other then that, I want to get a USB 2 header -> micro cable, and if I can't find the USB 3 header -> USB 2 header adapter that came with my mobo, one of those too. But, well, progress is being made. Slowly. Lazily. I'll get there eventually.
And then all I'll have left to do is shove the new HDPLEX AC/DC in it when it comes out, to make the whole thing brickless, and I'm good for a few years...