Help: Looking for Mod Pointers (Domain Aliases)

[Red Fox UK]

Hello There,
I am basically looking for pointers/advice on how to go about starting to write a mod that can do the following (I am a PHP developer, so just looking for vB modding advice):

Basically, we have the one forum install, at one location with one database - however the domain it is hosted on has multiple domain aliases pointing at the forum, IE:

mainforum.com
domain2.com
domain3.com
domain4.com

The purpose of this is to share the database (and users), now the main forum has lots of forums setup, but what we need to "mod" is that if a visitor enters on mainforum.com, all forums can be accessed, but if they enter on domain2.com, only certain forums can be accessed. The same should apply in search results, and all over the board...

Basically, its "dynamic forum access permissions, based on domain name"

Any takers? I imagine its quite simple as the vBulletin core is pretty easy to inject stuff into for a global modification!

Thanks in advance,
Warren