The DO schools use the AACOMAS system.
First, I don't think every DO school has a deferral policy. I think DO schools are wise enough to know that some people are sneaky enough to do what you described so they don't allow deferrals.
Second, I believe it is unethical to do what you said. It's just wrong. In the case that the particular school has a deferral policy and you don't care about the ethics of the situation, the school may ask why you are deferring and have certain rules. Certainly for MD schools, you cannot apply to other medical schools while deferring. This may be the case for DO schools also including crossing over and applying to MD schools.
What you should do is specifically call the school you want to defer at. If they allow deferring, ask the rules of it and see if they say you can't apply to other schools. If they don't then go ahead and apply MD.
I hope you do not choose to apply again to MD schools though. You will be wasting a year, and having looked at many DO match lists, I would say it is fairly easy to specialize as a DO as long as you're not talking about derm, optho, or other highly competitive residencies. You will also go through the pain of wasting all that money applying again and all the problems that come with it.