Hello,
You should be able to save yourself a bit of hassle by holding 'N' at startup which will make a NetBoot capable machine check the network for a NetBoot server. Alternatively, if the machine cannot find an OS, it will look to the network to find one.
Although your script is useful if you want to explicitly specify a NetBoot server from OF, that can be useful if you have more than one on your local subnet.
There are no OS 8/9 capable NetBoot servers as far as I'm aware. The technology was introduced with Mac OS X Server 1.0 (The Rhapsody original release), hence why OS 8.5 era machines supported it. There is a 3rd party utility which is now freeware which works on OS X which allows you to host Linux systems as well as OS 9/X called Boot Mania.
Also, word of advice don't try and host OS 9 boot images from 10.5 or later, it just doesn't appear to work. I think it's an incompatibility with AFP in Leopard which OS 9 depends on for NetBooting.
In respect to unsupported hardware, you need Open Firmware at the very least to NetBoot. Although PCI OldWorld Macs did have primitive OF, I believe it only works as far as initializing the PCI bus and not much else, Mac OS is still booted using a Hardware ROM. All New Worlds should support NetBoot barring maybe some of the very earliest models? In relation to unsupported G4s, it might work? I'm planning on testing it, but you will need to insert the modified Mac OS ROM into the .nbi image. But it *should* work.
Hope this info helps!