Mention a potential adviser in Statement of Purpose who is in another department
I think that in general this would be fine, but don't give the impression that working with him or the other lab/prof is, in any way, a condition of your acceptance. You want the department that you are applying to to be interested in you but not to give others the impression that you are inflexible in any way. But stating that you have a well worked out plan and have at least partial buy-in on it from a professor shouldn't hurt you, and might help. 

Your first goal is to get accepted, then work out the details. Make it obvious that your background supports your SOP goals.