On Google's whitelisting page it says:
PLEASE NOTE THAT YOU MAY NOT SHIP YOUR GOOGLE CAST APPLICATION WITHOUT A WRITTEN AGREEMENT FROM GOOGLE.
So my question is, can you publish your app with just the preview SDK? Or is this message basically saying "You can only publish the app right now if you are working with us(close communication), for example, Netflix."?
Nothing is stopping you from publishing, except that virtually no one will be able to use it since they'd need to have whitelisted devices and host your application somewhere. That being said, there've been a few people in the community showing off demos of things they've run up on github for people to poke around in.
As long as you don't need a mass audience and just want to share with other devs, then go ahead!