Why not try a "soaring regatta"? Gliders types that commit with say more than five entries get their own class and are scored as a one design. If fewer than the specific number of like sailplanes want to participate, they could be scored in one or two handicap divisions based on performance to round out the field.

There are plenty of less than state of the art racing machines that may have owners who would like to meet, compete, and compare notes on their particular model sailplane. Assigned task could be preferred and set according to performance of the sailplane/pilots in each class. Many of the more popular model types have newsgroups so this could be discussed among themselves to see if there is enough interest. I'm sure after an initial contest or two it would become apparent what the more popular designs will be. Maybe a poll could be taken to find out if there is enough interest in this concept before a contest organizer sticks their neck out too far.

Seems I remember a gathering a few years back of Libelle's at Air Sailing that was quite popular and successful.

Barry