Are you serious? An actual, practicing attorney told you this?
Time for a new lawyer
http://www.snopes.com/legal/postmark.asp
tl;dr Don't do that.
Think of some electronic equivalents of sending yourself certified mail: save your idea on Google Docs; email it to yourself and your friends; check it into a private Github repo and save it to Tarsnap (both cost $$ unless you're already using those services). You should flesh out your idea in great detail, as detailed as a patent. If you take all these steps, you should be able to successfully challenge any later patent on the idea.
But, if you want to be able to sue anyone who uses your idea, you're going to need a patent. That's what a patent is: a license to sue. So if there's a good chance that someone will use your idea in the next few years, filing for a patent now will save you a lot of headaches. Remember that it takes years for a patent to be approved.
The point of registering with a publicly known entity like the WGA is that it may prove a date for the prior art.
The specific issue here is whether the WGA's registry is sufficiently accessible that it constitutes a "public" publication.
The answer depends on what exactly is registered, and whether the idea is ever made public (i.e., internet blog, scholastic paper, conference presentation, etc) or is privately known to the persons seeking the patent (i.e., pitching this very idea to the people who later try to patent something based on your prior art, etc).
The WGA registry provides proof that something was registered on a given date, but does not provide a public search engine for searching the contents of registered works. So, by itself, the WGA registration would not provide any benefit unless you're trying to show that the patent-seeker had private knowledge of your prior art on a given date.
Definitely not. The WGA registry is confidential.
But if you really want to have a hope of preventing someone else from patenting it, then you need to publish.