From looking there is a Dev signing key in the source code on the Moto website.
Or at least there was. I have a copy of that key, and I think that is what he is talking about. What I think he is saying is that the backflip will except roms signed with this key. But I think moto might have pulled the key down, but I still have a copy. That being said, I don't know enough to try it. Also, from looking at the rom notes, I think they crippled source so that if you load a custom rom you break ADB software loading.