You cannot publish forms via webservices. Webservices are about function calls, not about user interaction.
You could publish each induvidual stap of the wizard though, but you have to write the UI (logic) in the target application.
Probably I have not been clear, but the customer has chosen for Mendix to maintain the complex logic and connection with the backoffice software. The websites (many brands) are maintained by a company specilized in end-user design. Maybe from Mendix perspective this should be merged into one system, but that's no option.
I made a prototype that has each form as entity, published thru a webservice. After completing a step the next step is returned. Internal entities are filled after completing the wizard. This made it a general layer for exposing business logic.
Mm.. guess i still do not understand your question. In the modeler you can publish any microflow as a webservice, which can be called by any system with decent WSDL support. So in your target application you need to program the UI, and the invocation of those webservices. I don't think Mendix can ease this anyhow.