Solving the holiday boss summon problem

We've heard about a lot of problems with the Headless Horseman summon this year already (and Coren Direbrew had issues as well), but Kimberly over on WoW LJ has the worst story I've heard so far -- a group asked her to heal, claimed they all had summons, and then after killing the HH with her summon, ditched her, presumably to find some other sucker with a summon. That's not just scamming someone into taking you along for a round of summons (as we heard about in Guildwatch yesterday), it's actually scamming someone out of the rest of their chances, because, assuming Kimberly is honest for the rest of the day, why would another group take her?

So this is clearly a problem. Let's fix it for Blizzard: how can we make sure everyone has five chances a day at summoning the Headless Horseman (one per day per group member) without ripping off people for helping their guildies or friends? Basically, we need a clear way of seeing whether someone has a summon or not -- maybe a debuff that resets at midnight every night, or a changing number of candles on the pedestal based on how many people in the instance can still do the daily quest? Right now, there's no accountability at all, and with droprates the way they are, getting screwed out of another chance at the HH (or even four chances, in the worst cases) can make the difference between having the achievement and not.

Any other good ideas on how to solve this problem and keep players accountable? I'm sure players with less than upright morals have no problem claiming their brother used their summon or lying to steal the summons of others, but it would be a lot more fair for everyone if the possibility for scamming wasn't there at all.