I eventually solved this by saving a copy of the WSDL file locally, and, since JaxRpcPortProxyFactoryBean expects a java.net.URL for the wsdlDocumentUrl property, had to set it with a path like file:///c:/.../blah.wsdl.
This isn't really all that desireable, I would hate to have to put a file:/// URI in a Spring context file that might be deployed on a server, especially on a different platform - seems odd that this class behaves this way.
I'm guessing most people aren't using Spring aren't using JAX-RPC anyway.