If I am a member of multiple networks, how can I force my Mac to connect to the Internet through one specific network?
You can accomplish this without having to change between both networks. Connect to both networks. From the command line run these two commands.

    sudo route add -default *ipOfEthernetGateway*


That will set all traffic to go through the work ethernet link.

    sudo route add -host SOAPserver.com *ipOfWIFIGateway*


That will set all traffic that needs to go to the SOAP server to go through the WiFi link.