What if, when signing up for a quest, players are asked to post the names/dates of the last 3 quests they participated in. Or ask players not to sign up for a new quest if they have done one within a certain time frame? Both of these suggestions would help the DMs to know who on the sign-up list has done a quest recently without having to do a lot of research and give players that haven't been involved in many quests a chance to get priority on a quest roster.