Conditional SSL client certificate request with nginx

Is is possible to make the SSL client certificate request conditional depending on the IP range of the user with nginx?

ssl_verify_client optional always sends the request, but does not fail when no client certificate is provided. I want to make the request itself conditional.

The problem: I don’t want the users within the intranet to be bothered by the browser asking for certificates, but only if they access the page from outside.

This fails:

if ($intranet != 1) { 
  ssl_verify_client on; 
} 

Error:

"ssl_verify_client" directive is not allowed here