Thanks Aaron that did exactly what I wanted. I do plan on applying findclass to it, but I wanted to do it the hard way first to make sure my concept was sound.
Another question it brought to mind is whether or not this is the best way to go. A different theoretical solution would be to create pools for each new domain added which point to the same IP address, but on different ports (between F5 and IIS). The rule would instead call a specific pool based on the domain name found in HTTP::host. On the IIS side, I imagine that each domain would have its own web site, with the corrsponding port identified in F5's pool. It seems like it would be more administrative overhead, but there might be other reasons that this could be preferable. Any thoughts?
Thanks again for the solution!