Here's a question posed to me many a moon ago: Do you have the legal right to file a treatment/concept with the WGA using CBS's intellectual properties? The argument being that since you used their intellectual properties in a registered work that you are laying copyright claim too, that you are in a roundabout fashion, claiming to have a stake in their copyright. Which could be viewed as a violation of their copyright; seeing as your were not employed to create the work.
Well, I think I can unknot some of that... Registering with the WGA is not registering copyright.
Copyright can only be registered with the Copyright office. And I don't think they would give you a copyright on a treatment for Star Trek, as CBS/Paramount control all of that.
Registering with the WGA is basically saying this draft was done at this point in time. There's no claim of copyright.
ALSO: when you sell something to a network or a studio, they buy the copyright. It's their property totally. They pay you lots of cash for it, but, it's theirs to do as they see fit. So, then they could register THAT at the copyright office.