Redirect for Search Engines (googlebot et.al.)

[Malte Sussdorff]

I want to use a (slower with limited amount of threads and  
connections) backend server just for search engines which will not  
have an impact on the users of the site (as they are using a  
different upstream).

Will something like this work ?

location / {
	if {$http_user_agent ~ Googlebot} {
	 	proxy_pass http://search_engines;
         }
}

Furthermore, is there a way to make this global for the server across  
all locations (as I have 25 of them, so I would have to add the code  
on each). Last but not least can I provide a variable with a list of  
user_agents (as there are a ton of search engines)?

Thanks in advance and sorry if this has been asked before and I am  
unable to find the answer using google.