Django-nonrel is in somewhat rough shape and your code ends up coupled to the platform anyway. No plausible path off of App Engine except a rewrite. So better keep it small.
I believe AppScale is a plausible way out. I tried to use Typhoonae, but couldn't make it work.
I've been playing with the idea of building a App Engine gear able to run Python apps on top of OpenShift, but my day job is interfering with my hobbies...