Ehm, wait but you're just requesting config url from the app, it's up to app what will it give to you - http://configure.myapp.com/pebble.html or file:///tmp/configure.myapp.com-pebble.html - browser will open both url disregarding whether you're online or offline.
Of course two issues here - you need either to prepare that path (eg. copy the config folder with all resources to tmp) or to provide custom schema (eg. pebblejs://open/) which will in our case be mapped to something like .local/share/rockpoold/<mac>/apps/<sid>/config_page