A token system would work very easily.

Clear Turn 6 and get a drop? No token
Clear turn 6 and get no drop? token given upon exit
Take 4-8 turn 6 tokens or whatever seems applicable to trade for one piece of Turn 6 gear. If you cleared it for 2 months without a single drop you've by all rights earned at least one piece of something you want from it. If you get the drop you wanted sooner, cool you can buy something for an alt job to help your group or a piece of gear to vanity.

Just make each turn have its own token that only buys things dropped from that turn. Put the npc in the intermission map we get dumped into between turns or something. Ancient Allagan Vending machine