It will be impossible to deliver a worthwhile UX if the details of the product are obfuscated from the design team. Find a reputable product design professional willing to execute an NDA.

Figure out a compensation scheme that makes the both of you comfortable - but make sure his skin is in the game and he's not just burning hours. It doesn't have to be equity - maybe a big payout once you hit a certain critical mass of users.

I'd be glad to put you in contact with some folks that may be interested. I'm at tom [a] thegrif [dot] net. Good luck :-)