Oh good question! The custom network stuff is not well documented. You can expose an app outside its custom network through Flycast. You’ll want to allocate an IPv6 on the target app, specifying the network you want it to be accessible from
fly ips allocate-v6 --private --network <custom_network_name> --app <target_app>
If you want it to be accessible from your org’s default network, you can just omit --network.