Love, love, love this plugin. It's so useful for my day to day when developing locally.
I am, however, having an issue getting it to work on my staging environment using a subdomain - separate machine/IP from production. I've set my wp-config.php with the following constants.
define('UBP_IS_LOCAL', true );
Seems like the 404_template hook isn't even firing. I'm running NginX, APC, Varnish and then also running Cloudflare on the production server (AWS Ubuntu - elastic IPs).
Any help is greatly appreciated as I would love to be able to use this on staging.