Choosing servers based on IP address

[Franks Andy (IT Technical Architecture Manager)]

Hi all,
  Quick question - can anyone think of a way to change a server's weight
based on some criteria, for example source IP address? It would be so
useful when dealing with a common service that has two distinct sites,
and rules in place that stop access to resources from the "wrong" site,
like Exchange (where you can't access your mailbox from the wrong
site-based CAS server).
I found a patch that does dynamic server weighting at a preset time for
all clients, but not a per-client weighting scheme.
If I can't do this, could I do it with LVS does anybody know?
Thanks
Andy