Sorry for the delay! I tried it on a whim with one of my old PS1-to-USB converters. The dial doesn’t get picked up at all in the standard mode, but as you said it did get picked up in the neGcon compatibility mode. The problem is the neGcon assigns its twist function to a joystick axis, and as a result that’s what ends up getting assigned to the dial in that mode. This makes using it as a spinner a no-go, as you end up with a floaty steering wheel with lots of travel.
Bit of a shame, but on a related note a friend has apparently picked one of these up. I’m waiting to hear back on how it works with MAME et al, but seems like it might be a nice oddball little guy to have around!