This new "Bitcoin Beacon" paper has some answers to the OP:
http://arxiv.org/abs/1605.04559

Maybe in your context people already trust the organizers, so you don't need public verifiability of tiebreaking, and then commit/decommit (with or without penalties) among the organizers is already ok, but the beacon will only help of course (described in section5 of PDF in the arxiv link).