altboot uses kexec to start any kernel of your choice (even openBSD)
i think that this is perhaps more trouble than it is worth, but its been a long time scince i had a sharp rom or simmilar on my Z and i cant remeber what the flash layout is like
if it is posible then you would need the rootfs for OZ on an SD card or CF and have altboot installed. you would also need the cakco or sharp rom kernel stored somewhere (perhaps on /dev/hda1 in a /boot folder)
tell altboot to boot off of CF/SD to boot into OZ or tell it to boot off of a microdrive for cacko, i cant remeber the specifics of altboot as i have never dual booted but this is the basics required to do it