JoelKatz's offer still seems like the easiest way to build more trust. Legally binding non-disclosure agreement forms are freely available on the web, and I'm sure there are forms that can be filled out and signed electronically. What could be easier than that?
NDA could be and had been ignored in some cases. But given that Joel has his face there, and if I'm not mistaken, him being the same JoelKatz active in other projects, I don't think he would want to his name and reputation just to steal a machine. So asicminer really has no reason to fear letting Joel Katz know just a bit of information, unless he knows there isn't a working prototype of any sorts.