I know this is an old post, but I need to find a solution too. 

 

Another solution I thought of for the original questions was to have a k2 user group that only allows users to publish their items at 'registered' access level.

 

This is kind of similar to having extra categories as Antonio suggests but without having extra categories. I just don't know how to implement that.

 

Any ideas?

I have implemented this mod, if someone wanted this please contact me. $50,this zip file contains everything you need to view and achieve this withouct hacks to original coding.