Can't escape "/" characters in parameters of SSO REST callback in JPP 6

Issue

The integration between CAS and the GateIn IDM included in Portal involves the REST authcallback URL http://localhost:8080/portal/rest/sso/authcallback/auth/user/password. This returns true if the username and password combination is valid.

However, we recently had a customer who could not log in since his password contains a "/" character. Is there a way to make the CAS callback call work with passwords containing a slash?