It's less than ideal, but if you disable wireless connectivity and only load new books on via USB connection (it shows up as a generic USB volume) I'd imagine you're safe, as that should theoretically completely stop the device from calling home.
This is exactly what I do. The device doesn't have the password to my wifi so it can't phone home. I have no need to connect it to the internet — I just transfer files from my computer over USB. Couldn't be happier with the setup.
I have owned a Kindle since GEN 1. I have always added my own DRM free books. I would say %80 of my library is DRM free. I rarely turn the WIFI on. There are several pieces of software than can convert files into Kindle readable formats.