I don't think I've ever seen one. Even makers that tend to have a machi on the rest of their traditional knives and even their western style knives (wa gyutos, etc.), don't use a machi on their debas, that I have seen. Maybe it has something to do with the knife being used for heavier tasks and the machi is considered a weakness for such work?
Maybe, Jon can chime in on this one. I don't think having a machi on a deba would make it weaker. Like you said, they are so thick that I don't think banging on the spine with a little machi would cause the knife to snap in half. Maybe its just how they are traditionally made?
I'm taking a guess: AFAIK the function of the machi is that you can take off the handle for polishing or whatever and each time you put the blade back on the handle it'll slip a tiny bit further into the tang hole. A deba by design has a shape of the tang that allows that without adding any machi.
traditionally deba dont have machi... that being said, i've seen a few that do. For example, Suisin's Ginsanko Momiji Funayuki has a machi... but its a funayuki (a type of deba)... probably not what you're looking for. As far as hon-deba go, having a machi is very uncommon.