The current Session interface can only be used smoothly with HttpSession since it can only return HttpSession. This means that using Tomcat to make an implementation of the SIP Servlet API would require a hack on top of the Session interface. I would like that we change the Session interface to return a ServletSession interface that contains the methods that HttpSession has in common with SipSession and SipApplicationSession to enable a better and more smooth implementation of the SIP Servlet API.
I have just made a patch that adds the changes I sugested.
The consensus of the Tomcat community seemed to be against your proposal, sorry.