Post-network, pre-mount commands in initrd?


I was hoping to build a Raspberry Pi 4 image that would download a kexec payload and boot it, only to find out that kexec doesn’t really work with the rpi4/kernel.

So, instead, I’d like to keep using a single kernel on an SD card and have it download a squashfs in initrd and mount it the same way a netboot store is normally mounted, with the tmpfs rw/workdir on top.

However, I’m not quite sure how to achieve this. Is this possible with the currently exposed options? Can someone point me in the right direction?