I think being part of the setting/lore would be great, but not sure if I would aim for quest related material. Put an Order building/fortress/monastery/mansion somewhere and call it a day for starters. If they need plot or story hooks some day, they can always return to it and expand on the idea.
Maybe quests related to uncovering the lore behind the Order? "I have been researching a long lost order: The Obsidian Order. I need your help in uncovering the mystery behind this order, and how it vanished." Then it goes into tracking down little pieces and putting together the clues to what happened. Maybe not in the first game, but maybe in the expansion.
As for me, I wouldn't mind anything, but a mention in the game (through lore or other means) would be appreciated, but not necessary. Just donating for the cause is enough.
(I did vote, and I have signed up to join after donating the $8. Just waiting for the tag.)