I was in a similar shell in the autumn of this year.
To be able to lot an item you needed to meet 2 criteria:
1) You had attended the previous run (which was normally farming)
2) You had place the item on your priority list (posted on a website before the run)
Whoever placed the item highest on the list (1-3) got to lot, with anyone else who put the item at equal level.
Seals were kept by the organisers, money drops were sold to buy new pops.
The system was very effective when we had 10-15 good people turning up for every run (ran twice a week), as it was a group of friends that were used to playing together and it worked well.
The group grew into Friends of Friends as we wanted to do Kirin. At this point there was some friction, where the some 'founding' members thought it was unfair that their previous efforts were not accounted for and that the new members could lot against them.
Around the same time, some people who only wanted 1-2 items, basically finished with sky, they didn't need any more so attendance from some people started to drop and we had trouble killing bosses. At which point the organisers got bored of dealing with mini drama and low numbers so called it a day.
If you are looking to do something like this there are 2 things I can suggest:
1) have a fixed rooster of people, and fix this early otherwise you will get into the new vs old member argument
2) Set a fixed length of time, our original plan was to do it for 2-4 months, basically keep going until everyone had what they wanted. But this caused trouble as those that had already finished didn't want to hang around "forever" waiting for the rest to get there drops, so if you have a fixed finish date then you can manage this a lot easier.
If people don't complete the gear they want within the fixed time then I would open the rooster and start again for another fixed period.