I hate that clients think the most important thing for a demo is UI. Working Application > Nice looking application with no functionality

"Working" can be a loose term for a demo. Might be your only opportunity to wow them and convince them that your solution will be a good one. So, UI is the most important part. But as long as you have a clear idea of how any mock ups could be properly developed within any likely constraints of working with these people, it's fine to use a bit of smoke and mirrors.