server {

  <snip>

  resolver 10.0.0.2 169.254.169.253 valid=5s;
  set $mybackenda some.aws.elb.com;
  set $mybackendb other.aws.elb.com;

  location ~ ^(/assets)?/mybackenda {
    ssi on;

    proxy_pass http://$mybackenda;
  }

  location ~ ^(/assets)?/mybackendb {
    ssi on;

    proxy_pass http://$mybackendb;
  }
}