e.g. a sample app using a service called foo via a URL http://foo/ which we can then point /etc/resolv.conf at the OpenShift domain for the namespace of the app.

It looks like we could expose the LOCALDOMAIN environment variable to be $namespace.vagrant.f8 or something to hopefully get the process resolving DNS names within the Kubernetes namespace
http://man7.org/linux/man-pages/man5/resolv.conf.5.html

though this will require sshd configuration in the VM's to allow LOCALDOMAIN to be overloaded. See https://github.com/fabric8io/kansible#kansible_export_env_vars for more details