Hiring any talent is usually done without exposing the goals of a project to someone - you may just need to expose the tools.

Interviewing by phone has not been any different than interviewing in person, in my experience. Except that I don't need to wear a suit. Come up with your questions, ask them, and then roll the dice. Same as normal. ;-)

You obviously cannot ask them to provide code on the spot. But, if you get them to email their portfolio to you, you have much more luxury outside of the interview than inside anyway.

And I think there's someone here who is offering a list of interested monks to contract work, which sounds like what you're looking for. That may be a place to look - you can view their contributions to perlmonks prior to interviews to see what you think of their skill without even mentioning the word "interview".