For non-open source stuff, those pesky NDAs say "You shall not pass !!!". I think it's a very good idea, still.
My two cent is, the reverse can be applied by showing the interviewee some source codes/designs/product.Good programmers definitely cringe at the sight of bad code and excite at sight of good stuff. Same can be said for designers with...well, designs. Rather low cost consulting too for the company who does that. Win-win situation.