I’d say bring your own keyboard or laptop and be honest with the interviewer: you switched to Dvorak for comfort and productivity and you think you’ll perform better at the interview if they allow you to use that layout, possibly with your hardware. I don’t see how that could make you look like anything other than a conscientious professional. It’s about the attitude: will you attempt to create the best conditions for productivity and success and, failing that, can you make the best of what you are given?