TMDA exception lists

I have a centralized mail hub right now, that is running TMDA nicely.

However, I am planning a distaster recovery scenario witha few 
replicated mail stores, and thought of an idea of having the mail hub 
split the mail via procmail into two delivery addresses, one for the 
live mail store and one for a backup copy or standby machine.

Problem is with this idea, I need to do mail forwarding.   So, if 
someone internally uses the mail hub to send mail if a machine is down, 
they get challenged, which I haven't been able to find in the 
documentation how to turn off TMDA selectively for my internal mail.

Obviously, I could put my domain in the domain senders file, but that 
won't work well as spammers use my return domain sometimes to send mail.

I was thinking of manually putting everyones specific Email address into 
the confirmed file inside my domain to solve this problem.   That way 
everyone would be confirmed, and could use the mail hub to send mail and 
spammers wouldn't be able to use my domain carte blanche at least.

But it bothers me that if they collect your Email address then they will 
get through as well, just less so if they send it too a user in the 
confirmed file.

Any suggestions?

-gc