create a new subvolume as a snapshot of / mount the new subvolume on /mnt/ pacman -Syu --root=/mnt mark the new subvolume as the default one
In the initramfs, you need an emergency facility to let the user list the available subvols (btrfs su list /) and choose one to mount (which would require adding subvol=/foo/bar or subvolid=12345 to the rootflags).
And don't forget about /boot, or any other relevant btrfs mountpoint.
How much space does a subvolume/snapshot require? Will you snapshot all btrfs volumes, or just system-upgrade related one. For example, /home and /tmp vs /var and /etc. Also this necessitates a subvol flag per partition - could be messy. Or is it a bad idea to allow to mix &match upgrade subvolumes - which would require keeping all the subvolume IDs synchronized? It seems logical to swap the order - create a snapshot of the current system which will then be upgraded - especially for security upgrades which need to be effective immediately, unless it is possible to remount a volume without rebooting. It seems that *pacnew and *pacsave files will no longer be required. Just some thoughts, Yclept Nemo