Thank you for your praise.
Unfortunately, our current reverse proxy interface just pass the original request though, the rewritten URL cannot be applied to it yet.
Instead of using lighttpd with SCGI, maybe you can try Rails on LSWS via FCGI interface on the backend, just make sure to turn off the persistent connection in the FCGI configuration, as Ruby's FCGI interface does not support persistent connection yet. Just a recommendation. :-)