CAS has a built-in mechanism to send responses via HTTP POST instead of HTTP Redirects. To enable it for your particular application (i.e. PeopleSoft), you should redirect to the CAS login page with an additional parameter: method=POST so your URL might look like:
This is determined by the spring webflow as such:
Relevant classes are those that implement the 'WebApplicationService' interface. The final view that is rendered by CAS and does a POST request is located 'casPostResponseView.jsp'