yes, maybe is better to do it on chroot cause of trouble at reboot time. Systemd will be removed and the machine will not find the good soft from systemd to properly shutdown the machine. Note this is possible but require more knowledge about how to unmount partition and synchronize disks.
Furthermore, depending of the installed system some extra configuration can be needed.
Anyway, this is the general way.
1) append pacman.conf with obarun repo :
https://wiki.obarun.org/doku.php?id=repositories
EDIT: edit it and comment all SigLevel field
# SigLevel = Required DatabaseOptional
[obcore]
# SigLevel = Required
Server = https://repo.obarun.org/obcore
[obextra]
# SigLevel = Required
Server = https://repo.obarun.org/obextra
[obcommunity]
# SigLevel = Required
Server = https://repo.obarun.org/obcommunity
[observice]
# SigLevel = Required
Server = https://repo.obarun.org/observice
2) synchronize and install obaru-keyring
# pacman -Syy obarun-keyring
EDIT: regress to the previous state about SigLevel in your pacman.conf
3) update your system (do to combine this step with the installation of obarun-keyring)
EDIT:
# pacman -Sddu
4) install pacopts and check if packages installed on your system and found at obarun repositories really comes from obarun repositories
# pacman -S pacopts
# pacopts origin obcore <- do the same for obextra, obcommunity
follow the instructions given by pacopts if necessary.Note this is an extra check and pacopts should not find anything anormal.
5) remove systemd components
# pacman -Rdd systemd systemd-libs systemd-sysvcompat
6) install the marvel
# pacman -S skalibs execline 66 s6 s6-rc boot-66serv tty@ -66serv dhcpcd-66serv
7) create a tree for the boot and populate it
# 66-tree -n boot
# 66-enable -t boot boot
configure your /etc/66/boot.conf to suit your needs
8) create a root tree and populate it with basic services
# 66-tree -ncE root
# 66-enable tty@ tty1 tty@ tty2 dhcpcd
This will give you a operational system with 2 tty and dhcpcd for the network enabled at the next boot. Obviously install the desire services to suit your needs
You are done. You can reboot.
Take a classic archlinux ISO, open a VM, boot it with the archlinux ISO and try the above procedure :).