There is actually a way to do this fully in process. I think Rob Pike initially build this, and it's being used in the App Engine framework (or so I hear...).
It looks like he takes the same approach as Revel. From the comments:
// Devweb is a simple environment for developing a web server.
// It runs its own web server on the given address and proxies
// all requests to the http server program named by importpath.
// It takes care of recompiling and restarting the program as needed.