@crackulator
I worked for the same purpose for several days, and finally I succeeded.
In the <boot partition>/ramdisk/scripts/halium, you can find this:
echo "/dev/root / rootfs defaults,ro 0 0" >>$FSTAB
That's where fstab is being rewritten.
See github:
https://github.com/Halium/initramfs-tools-halium/blob/halium/scripts/halium#L315