The session ID is nothing but a hash key for sessions. From the way the docs read, the actual session map is common to the entire Tomcat container, not one session map per Context. Which probably has to do with the fact that there is an option that permits a session to be shared across multiple webapps. So I can see where things would get messy if apps didn't have a common ID format.
Customer surveys are for companies who didn't pay proper attention to begin with.
subject: Set sessionIdLength just for one application