wildcard VS and SNAT

Certainly the solution you are trying to achieve is possible. Your current approach needs some tweaking though in order to get it to work. You cannot make a comparison of a raw IP address with a network address range without incorporating the IP::addr command. For example [IP::addr [IP::client_addr] equals 10.0.0.0/8] checks to see if the client's IP address is in the 10.0.0.0/8 network. It will probably be easier to do this check from an IF statement rather than a SWITCH. Something like this perhaps:

when CLIENT_ACCEPTED {
    if { [IP::addr [IP::client_addr] equals 10.0.0.0/8] ||
         [IP::addr [IP::client_addr] equals 172.16.0.0/12] ||
         [IP::addr [IP::client_addr] equals 192.168.0.0/16] } {
        snat none
    } else {
        snat 8.8.8.8
    }
}