Hi… The following older thread has a workaround using Flycast:
https://community.fly.io/t/create-postgres-cluster-in-a-custom-network/23669
(One of Flycast’s less well known features is that it can be made to bridge networks.)
Overall, though, I think a generic app would be better. The fly pg commands themselves are slated to be removed eventually. (That was true last I heard, anyway.)
Hope this helps!