if alpine linux didnt upgrade linux-rpi (netboot kernel version is the same kernel version for that release),
then just add zfs, zfs-scripts (required for compatibility.d files), zfs-rpi (zfs-lts for other devices) it is done!
if you boot with modloop enabled, simply do:
/etc/init.d/modloop stop; rm /lib/firmware /lib/modules
https://dl-cdn.alpinelinux.org/alpine/v3.21/releases/aarch64/
as of writing, v3.21(.7) includes openzfs 2.2.
you can refer to zfsbootmenu guide for mkinitfs, needs zfs zfshost?