I am presently using nginx like a transparent proxy for apache. I must use nginx + fcgi(php) but need mod-rewrite support.

Is possible or perhaps is there a different to my worries?

To only one site, would litespeed be a highly effective solution?

nginx includes a rewrite module: http://wiki.nginx.org/NginxHttpRewriteModule

It does not offer the same syntax as mod_rewrite, but has similar features.