On further examination, it seems that those very short pulses on the ZDET line that you can see on my attached image were just crosstalk from the LED line.
Which actually makes this even stranger! Why can't I see any signal at all on the ZDET line?
Attached is the Verilog code for my component.
The reason you're not seeing a signal from ZDET is because you have a failed solder joint on your PSoC chip.
Oh, cool, thanks. It works now.