My solution to this for my much simpler setup (two WAN links and only a few users) was to fix the Net Balancer so it works better.
Patch for b16 is at http://dl.dropbox.com/u/19663978/nb_qos_b16_patch.tar.gz
Patch for b14 is at http://dl.dropbox.com/u/19663978/ZS_nb_qos_b14_b.tar.gz
(No differences between changes needed for b14 and b15, so no b15 patch.)
It does lots of things, like allowing QoS to work with net balancing. But it also makes connections sticky so your banking transaction connections will all go out the same external IP address.
Install by getting to the shell prompt on your Zeroshell box, doing a wget to pull the appropriate patch from the above link. Use tar, etc. to unpackage it. There is a read me file in the package telling you how to do the rest of the install.
Fulvio has indicated that he will include this patch on some future release. But that has been a few beta releases ago and its not there yet.