"Fly-Region" on internal-to-internal http request

Most likely not, but the client-instance can explicitly set it as a http-header from FLY_REGION env var?

Similar to above, the client-instance can set this as http-header itself from FLY_APP_NAME env var?

There isn’t a http load balancer at all in the 6pn world. There’s a IP load-balancer if you allocate a Flycast 6pn IP and connect to the other app instances via it.