I finally have a working solution, using a P-channel JFET connected to the EN pin of the PowerBoost.
I am not sure if the resistor is necessary when using a depletion FET, but it seems to have eliminated a weird intermittent problem with powering off.
The only thing with this setup is that you can't switch off during early boot-up.
Here's a video.
Be sure to add the following to /boot/config.txt
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