I think you should be able to do

    boot enet:0 enet:0 -a

Which, assuming you have a BOOTP server serving ofwboot.elf and a DHCP
server serving a kernel, tells OF to boot ofwboot.elf via BOOTP off
the network with the args "enet:0 -a", which then tells ofwboot.elf to
boot via DHCP off the network with the args "-a", which will tell the
kernel to stop and ask for the root device rather than assuming it's
the same as the boot device.

If that doesn't work, you can try skipping ofwboot.elf and doing "boot
enet:0 -a" (with the BOOTP server serving the kernel), which should
work, although you won't be able to use DDB effectively.

You can also build a custom kernel, and hardcode the root filesystem
into it. (The kernel-building doc on the web page should explain how
to do that.)

-- Dan