For freelance service providers, this already exists. I do it with everyone who wants to hire me. No prepayment = no work. It took a long time to get there. It also took multiple times of forgetting this rule (and getting burned once again). Most recent write-off? Last month. :-(

Just ask. It is astonishing what the universe will deliver to you if you just ask.

Also, if the answer comes back "no" too many times, that is valuable intelligence gleaned from the marketplace. What you do with that information is up to you. Personally, I interpret that information as "I am attempting to compete in a commodity business; how can I reconfigure my services so I am not a commodity service provider?"