So why not use a cookie instead?
or better still, do the right thing and use a shared database for sessions.
If we are talking about decent scaling and reliability, then you want session and state info to be kept at the server to save on a) passing it all back and forth and b) validating it all.
On any UNIX system it is trivial to store the information in a DB that is shared between multiple webservers. There are lots of ways of making that kind of thing scale to very high levels.
Hidden variables are a sign that you haven't finished designing your application