Outbound SNAT w/ IRules

Yes, this is possible. You can also forward outbound traffic based on a particular source, I use this for return traffic in a firewall matrix. In your case, your servers are the clients, so this rule should work for you.

when CLIENT_ACCEPTED {
   if { [IP::addr [IP::local_addr] equals "A.A.A.A"] } {
      use snat Z.Z.Z.Z
   } elsif { [IP::addr [IP::local_addr] equals "B.B.B.B"] } {
      use snat Y.Y.Y.Y
   } else { use snat X.X.X.X }
}

Note that you can use a mask if it is a network destination, like this:

"A.A.A.A/24"