A feature to pin clients would be helpful, no doubt, but folks are indeed creating N apps in its absence (0).
Apart from fly-replay, one can also forward request to a particular instance with fly-force-instance-id (1). In a way, doing so (fly-force-instance-id) is better because the request ends up straight at the desired instance, instead of (fly-replay) being ricocheted off of another instance.
Speaking of ricochets, one can forward traffic to any instance over 6pn (2, 3, 4), too.