A bit more info on the app I'm thinking of testing with Flynn: each instance has a single dedicated resource (actually a psuedo terminal sitting on top of a command line tool). Any given instance is incapable of handling requests concurrently, they queue up at the instance. So the app's concurrency model relies on smart load balancing at the router itself. I've got HAProxy playing this role, but have done a bit of a hack job juggling the containers which implement the various microservices.
I'll give Flynn a go to see if the simplicity it gives me in managing my deployment is worth the routing tradeoff for now, but I would definitely love to see Flynn's router adapted as you describe.