Can I conduct psychology research involving my restaurant customers without their explicit consent?
What does your advisor thinks of that? 
What does your ethics committee thinks of that?

Because, as far as I know, every and all experiments that remotely involve humans need to be approved by an ethics committee. 

I'm from computer science but I had a few experiments involving people and that is usually the rule...

And remember, each country has its own set of laws, make sure you know yours!

Apart from that, my first impression was "bad idea". Even if it is, by some weird loophole, legal, it is not very ethical, at least IMHO...