I've put together an implementation of Luke Hubbard's RT on calling web services from the flow layer.
The code is available from http://wiki.apache.org/cocoon/FlowAndWebServices if you're interested in taking
a look.
The impetus for getting this going came from an application I was working on where I needed to direct the
flow of control based upon the response from a remote service, something not easily done using the SOAP
logicsheet or WebServiceProxyGenerator.
The code probably needs a bit of a tidy up, I'm not hugely familiar with the Avalon component lifecycle.  I also
lifted the CompilingClassLoader code directly from the FOM javascript interpreter - I'm sure it could be better
optimized for this purpose.