"Additionally, it is the only client that is able to use secondary protocols. ItĘs the need for secondary protocols that make the FWC necessary for apps like Instant Messaging, streaming media, FTP, etc."
Basically (and correct me if I'm wrong), if a game needs to use secondary connections, through the ISA server, you absolutely need to use the firewall client. SecureNAT won't work.
I don't know if you remove the secondary connections from some of the definitions and create primary connections for each port or range of port numbers, will work. I'll give it a try myself and see where it leads, but I don't hold up much hope.