Couldn't you use a cookie and map, rather than URLs? Sounds like users could end up on different rails back ends unintentionally based on what URL they use, with your method...
http://serverfault.com/questions/268633/controlling-nginx-pr...
http://serverfault.com/questions/268633/controlling-nginx-pr...
We want to allow our users to enable beta version for all visitors to their Mixlr page, not just for themselves.
But a cookie approach would be easy too - see my other comment for a simple example.
However I wonder if map is faster than using lua just for cookies. I'll have to test it. :)