Load balancing and specific interfaces

Hi everybody,

We have a Watchguard Firebox with one internal interface and two
external DSL interfaces for load balancing.
The problem is that certain traffic (1) needs to go through a specific
interface as traffic from external networks is not allowed.
I would have thought this would be a common scenario but apparently not
as even the 'experts' cannot figure it out.
Does anyone know of a way to accomplish this, even if its not spcific to
the watchguard firebox product it might give me an idea of where to go next.

(1) Usenet access requires that traffic come from a certain network for
it to be authenticated. I cannot use a service providers usenet server
without being on their network.

Cheers
JB