You're just going to piss someone off doing this. Nobody is going be comfortable paying for work that wasn't rendered, regardless of where the holdup was. If you're doing 50% up front, but haven't even finished the design and they're not getting back to you, then you have a massive net gain already. That's the reason payments are broken up like that, so you don't lose your ass. Telling them because they didn't get back to you that you're going to bill them for the rest of the project is just silly, and unethical.