You should then be able to access .internal hostnames (like the Redis app will have) locally (which of course otherwise would not resolve) as if you were a Fly app using the private network within that same organization.
@greg has the best answer. There’s a slightly simpler way that users Wireguard under the covers. Run fly proxy 6379: 6379 -a <name-of-redis-app> in one terminal, then you can connect to localhost:6379 from another process.