Sorry to break it to you, but you have totally missed the point. Arc is not finished, it is still an experimental language. There will be unicode suppport, but there are many core issues yet to be resolved. You shouldn't use Arc for actual projects right now anyway, as Paul already warned that future changes not only can, but will break old code. So this is not the time to worry about Unicode. It's a bit like building a house and worrying about the curtains while you're still stacking bricks...
Normally, you (or the language/framework you use) will set a cookie with a unique session id, then store the value in memory on the web server or in a database referencing this session id.
Then when the new request comes, the user agent (browser) will send the session id back to the server. You can then use it to look up the original value.
