I'm not a Weblogic guy, but I was looking over the 8.1 docs and it seems it implements J2EE 1.3 and JAX-RPC 1.0. Being that the case, the method of deployment of Message Handlers is not mandated by the spec. See JAX-RPC 1.0 12.3.2. That changes for J2EE 1.4 when webservices.xml is used.
There seems to be some good docs for Weblogic though. Take a ook at this one, which guides you specifically on designing, building and deploying Message Handlers with Weblogic 8.1: "Creating SOAP Message Handlers to Intercept the SOAP Message"
one is the parent doc. Lots of info.
Hope it helps and don't forget to share the results of your implementation.