While playing with the MediaMVP box, I discovered DVArchive. I've got no use for it as I don't have a ReplayTV box but it should prove useful for anyone that does. It allows you to pull recordings off of the ReplayTV box and serve them up via an internal UPNP server. As it runs entirely out of Java, it's pretty simple to set up and run.
The one thing that is hidden (left out) by the documentation is how to start the program: java -jar DVArchive.jar.