OK, I've investigated it here and it looks like we'll need to do further work to implement support for directly blessing a specified NetBoot bootloader over TFTP. For now, we can only boot from servers which are discovered automatically, from a NetBoot server on a directly-attached network (or possibly where there's a DHCP relay to the NetBoot server on a remote network).
I don't immediately see any workarounds.
Cheers,
--
Darius