Outline

Install Asahi
Add cryptsetup into initrd
Make initrd spawn a shell before mounting disk
Reencrypt the partition in-place
Configure grub and rebuild initramfs to open LUKS device during startup.
Cleanup

Steps

Install Asahi

Just curl https://alx.sh | sh and follow the prompts. Move on to the next step once you boot into Asahi.

Reconfigure initrd

Add crypt dracut module

echo 'add_dracutmodules+=" crypt "' >/etc/dracut.conf.d/crypt.conf

Add cryptsetup and dependencies

cat >/etc/dracut.conf.d/cryptsetup.conf <<EOF
# cryptsetup binary
install_items+=" /usr/sbin/cryptsetup "

# cracklib because cryptsetup checks the password using it
install_items+=" /usr/share/cracklib/* "
EOF

Make dracut stop before mounting root - edit the GRUB_CMDLINE_LINUX_DEFAULT entry in /etc/default/grub:

remove rhgb and quiet
add rd.break=pre-mount to it.

Rebuild the initrd

dracut -f

All should be set now, reboot away.

reboot

In the initrd

Hopefully the boot process should stop and you should be able to log in as root.

Follow https://davidalger.com/posts/fedora-asahi-remix-on-apple-silicon-with-luks-encryption/ starting from "Shrink the btrfs filesystem to make room for the LUKS header."

The arch-chroot utility is not available in the initrd, but we can work around that. Assuming you have your fedora partition mounted under $FEDORA_MNT you can do the following:

mount --bind /dev $FEDORA_MNT/dev
mount --bind /sys $FEDORA_MNT/sys
mount -t proc none proc

$FEDORA_MNT/usr/sbin/chroot $FEDORA_MNT bash

Cleanup

Edit the GRUB_CMDLINE_LINUX_DEFAULT entry in /etc/default/grub to not include rd.break=pre-mount and add back the previously removed options (rhgb and quiet).

Remove the rd.break=pre-mount from

rm /etc/dracut.conf.d/cryptsetup.conf

Rebuild initrd and regenerate grub config once more

grub2-mkconfig -o /boot/grub2/grub.cfg
dracut -f