It's actually really easy, there's rarely a reason for the initrd to be more complex than a single 50-line busybox script on an embedded device with built-in drivers.
I got an immediate answer with a solution from the upstream developer when I asked about it: I can't imagine how that could have been easier to solve. And it was also trivial to hack around by leaving the entry out of fstab and mounting it in a script.
That's not true, systemd has an explicitly documented and supported initrd interface: https://systemd.io/INITRD_INTERFACE/
It's actually really easy, there's rarely a reason for the initrd to be more complex than a single 50-line busybox script on an embedded device with built-in drivers.