Syslinux

Booting Obarun with Syslinux works exactly as the Arch Wiki describes. Edit /boot/syslinux/syslinux.cfg.

ro or rw on the APPEND line

66 boots either way. Syslinux entries on Obarun ship with ro, GRUB produces rw, and both systems boot. There is nothing to fix if yours says one rather than the other.

The difference is the root filesystem check:

APPEND	What happens
ro	The root arrives read-only. The boot sequence can run fsck on it, then the mount-rw service remounts it read-write.
rw	The root is already writable. fsck does not check a filesystem mounted read-write, so the root is not checked.

The shipped default:

LABEL Obarun-Linux
    LINUX /boot/vmlinuz-linux
    INITRD /boot/initramfs-linux.img
    APPEND root=/dev/sda5 ro quiet net.ifnames=0

and the other choice, equally valid:

    APPEND root=/dev/sda5 rw quiet net.ifnames=0

A ZFS root needs rw: without it the root stays read-only, and fsck has nothing to check on ZFS anyway. See GRUB for the longer version of this choice, and The boot module for FORCECHCK and the system-fsck service.

UEFI

Syslinux can boot a UEFI/GPT layout, but with limitations and enough extra setup that GRUB or EFISTUB are the recommended options there. See Manual UEFI installation, and the Arch Wiki on Syslinux UEFI systems.

Kernel command line and 66

Any key=value pair valid in /etc/66/init.conf can also be passed on the kernel command line, which is handy for a one-off debug boot:

APPEND root=/dev/sda5 ro VERBOSITY=4

See Boot output.