The PCB for the power button arrived some time ago, and recently the last of the parts made it into my mailbox — the pair of mosfets in a single package. So I went and assembled everything, and powered it up, and what do you know, it didn't release blue smoke. But it also didn't work — it was always on. So I went through the schematic again, and then asked on the hack chat channel for help. Rubber-ducking helped and I noticed that I have swapped the P and N mosfets in my design. Ugh. I also tried the PCB on the zero, and turns out it doesn't fit quite right — the HDMI socket is a little bigger than I thought and needs a deeper cut out. So here's a new version of the PCB:

Hopefully that one will work better.