You can use 'map' to set a variable based on several client browser values and use that variable to pass the request to whichever proxy or fcgi server.
Some reading stuff:
http://forum.nginx.org/read.php?2,194480,194990#msg-194990
http://www.cyberciti.biz/faq/nginx-redirect-backend-traffic-based-upon-client-ip-address/
http://www.kutukupret.com/2011/05/11/nginx-reverse-proxying-multiple-domains-using-map-module/
Some reading stuff:
http://forum.nginx.org/read.php?2,194480,194990#msg-194990
http://www.cyberciti.biz/faq/nginx-redirect-backend-traffic-based-upon-client-ip-address/
http://www.kutukupret.com/2011/05/11/nginx-reverse-proxying-multiple-domains-using-map-module/