Tim,
you are right, DO students do not have to partecipate in the MD match, although many end up doing so because of the limited number of osteopathic residency programs.
As for the rest of the info, it is a known rumor/fact that many people "work things out with the residency director at osteopathic programs outside the match". That applies to osteopathic programs, though, and I haven't heard it happening to DO students at allopathic programs. In any case, it is not legal in either match. I have heard it goes something like this: you go to an interview and then they tell you if you are ready to accept an offer from them on the spot. Apparently, some people have gotten burned by doing so because, unless you have a signed contract in hand, the program can withdraw their offer at any time.
You cannot match DO and then "dump" that program if you get into an ACGME program later. It is a big "no-no" and your original program can go after you for breach of contract. They know they "won't be squeezing blood out of a turnip", so to speak, so they can do worse. They can contact the ACGME program and let them know of the situation, so that neither will take you a this point. Very bad form, for you, to say the least...