Just a suggestion. Since the rules and participants are agreed upon from the beginning...it would make sense that the 'moderator' for the debate should be one picked/agreed upon by the participants. In that way, one can also pick individuals who are not regular mods...say...invite a specialist in the field (if they are willing to join this forum), or even tap regular members.
Perhaps creating a seperate subforum for formal debates, where all moderators who are open to the idea are there.
Also, have you considered defining/limiting the 'role' of debate moderator..as opposed to 'forum' moderator? Forum moderators can alter posts and their role includes functions that I don't think are usually in the pervue of debate moderators...to avoid wearing two hats...so to speak. Also, by doing that, the participants can also select non-mods for the role of debate moniter.