". . . Is this possible?"
Yes. Configure a workstation exactly as you would like it. Create a NetBoot image of that workstation using SIU. Save the nbi to the NetBootSP0 folder. Enable the image in the Service and tick the Diskless option. Choose any of the usual methods to use that image.
". . .and how can I do it without screwing up AST?"
You can have as many nbis as you like in addition to AST with no danger of screwing anything up. I'm currently looking at two servers that have approx 20-odd nbis being served, inclusing AST. Some NetBoot, some NetRestore and some NetInstall.
Just an update, I was able to get an image of 10.6 booting over the network, and it seems to work great.
The only thing is, I had booted the image on one machine to get it set up and install some programs, and then when I tried to boot it on another machine, I was brought to Snow Leopard setup, even though I had already set it up and everything. How can I get changes to "stick"? Or is that a limitation of netboot?
In the meantime, I'm going to try it again with an image pre-setup and with apps already installed to see how that works. But in the future, I'd like to be able to add things to the image without having to prebuild it locally on a machine.
Edit: I just realized you said initally that I need to configure a workstation just how I want it, my mistake. I'll give that a shot. So does that mean it's a limitation of netboot to not be able to add anything to the image, after it's been created?