Just a quick update in case anyone is currently building their own. I discovered last night that GPIO4 is not the best pin to use for the JFET gate. It seems to be because it's the GPCLK0 pin. The pin voltage drops to almost zero if the battery is low, preventing a graceful shutdown. Should be able to use any other standard BCM pin. See here
On the subject of PCBs, I'm thinking it might be an idea to have one version with integrated DPDT switch and another version with space for the original DMG-01 switch. Maybe I could incorporate both into one board. What do people think? If there's enough interest, I'll be ordering about 100 with surface mount components. Should be a very affordable part for any GBZ.