There was an old VB 3.x product called "Different domain, different style" that allowed us to specify a separate style that was called based on the domain in the browser request. This worked well for us, as we have six different sites that have some forums which are shared and some that are unique to each site. This mod allows us to run them all from one db and one set of code.
Also part of this project is a mod to display forums based on domain.
Right now we are looking for estimates on getting the work done. If it helps I can provide the code we used on VB to do the same.
Prior to beginning work on the project we will provide proof of a Xenforo license for each domain.
I'm sure you will have questions. Let me know what I failed to make clear.