Yea echoing Ignoramus you will need to do something at the phoenix/plug router level with fly-replay.
One suggestion was to do append a subsub-domain to assets like [vmid]. assets.app.com and then use the Fly Replay to tell fly to retry the request on the correct machine id. This should be fairly achievable via a plug and url helper.
We are currently moving our system to Apps v2 which is currently in pre-release which should deploy much faster to ameliorate some of these problems.