Only way I'd reduce it to a 0-cost is if you also attached penalty to the bonus: ie, you're Marked by the Power of The Great Bunny, and while some may scent the aura of power about you and pay heed (+1 to most social skills) those who recognize the source of your power rarely take you seriously at all (-1 to most social); or you're Marked by something great and nefarious- unless you're going incognito (no bonus), you only receive the social bonus w/ equally dark powers, or when using intimidate trappings (+1 to all social, situational)- with most others, you in fact receive a penalty to everything but intimidate (-1 to all social, situational... diplomacy tends to backfire. Badly.).
But at that point, you're still better off just working with compels to the high concept.
Edit: for the Great Bunny, I might consider that for a +0, all opponents aware of your supernatural-ness have their social defenses modified by Lore... the higher their lore, the less likely they are to take you seriously, and vice versa. No other bonus though.