First, don’t use SoapRPC if you need to create services that are interoperable with non-WM clients. Numerous posts on the reasons for this elsewhere at wMUsers.
The only way I can think of to support this is to create a container document that contains a document type reference to each of the potential doc types that might be received. Common practice is to configure each of these as optional and to add a text variable that is used to identify which of these are included.
Other than that approach of emulating the XSD choice model, I can’t think of any other way.