Hey Jamie,
I'm utterly stumped. There's a discussion in the forums here:
http://www.virtualmin.com/forums/message-view?message_id=14813
(Ignore the pictures and the stuff about him not having a shared IP option--that's a non-issue orthogonal to this problem.)
And I broke down the problem into little pieces to see if I could walk Benjamin through to a working setup, and I'm simply unable to make it work after half an hour of poking at it (I know I had this working in the past, but it looks like things have moved around some--some of the relevant options appear to have moved to Server Templates, and I now don't know what to put where to make it actually do something).
So, it's gonna take some documentation improvements at the very least, and likely some changes to the code to make it easier to understand and make work. This is probably the common use case in a large-scale hosting environment, and so it ought to be as simple as not doing it this way (or as close as possible). If I'm just being stupid and you think it is already sufficiently simple to use, I'll be happy to document, but you'll have to show me how to use it. ;-)
Pointers?