SquashFS+UnionFS as root filesystem?

What's an easy way to to use a union filesystem (read-only SquashFS + R/W overlay) as /? I'm thinking of using a custom boot script as the kernel init= argument. The script would mount the SquashFS + overlay, then chroot into the mounted filesystem and run the real /sbin/init... however I'm not sure how well this works with systemd init, and I'm wondering if there isn't a simpler option.