I just recently changed to using the NetSUS instead of Apple Servers for net booting. I have built a new image using AutoCasperNBI and I can successfully boot to it by holding ALT at startup and manually selecting from the available startup disks.
What I can't do is see it in "System Preferences > Startup Disk" which is weird and if anyone has an idea what could be up, I would appreciate it as it breaks zero touch imaging.