Well, if this is the case that would be a pretty deep architectural problem, I'd say, for a PaaS like Heroku.
I think it's pretty obvious that you need at least two layers of hierarchy for the routing here: One (or more) router forwarding requests to virtualized routers (per Heroku customer 'instance' or whatever that's called), which in turn provide the functionality I described in software. I'd probably use VMs running a specialized minimal linux distro for the per-instance-routers.
