It's an interesting idea, but for a game like PSO I feel using a point system would be more efficient and fair. I'll give a basic example of how the system could work:
A member can obtain points by participating in a hunt for an item. A standard party would consist of 4 people ideally (maybe less considering the circumstances). So in other words, a party would consist of 1 member seeking a desired item and 3 people helping to obtain that item. The member seeking the item would receive no points. The other members helping would receive points based upon time spent helping to find the item.
What happens if the item is not found?
The member seeking the item in question will not be charged any points. Based upon the time spent seeking the item, other member will be awarded points for their efforts.
How will parties be determined?
I suggest that before each night a poll/sign-in sheet be placed on the forums. Members wishing to participate can sign up ahead of time. There would a 1 hour cut-off prior to start time for leaders to decide how to split up participants.
How much will "x" item cost?
This is the difficult part, I would suggest coming up with a formula factoring in:
1. Rough number of enemy spawns during a level (the enemy that drops the item)
2. Average time spent to clear the level (including boss fights so helpers can also receive exp)
3. Drop rate
(I can help with this if this is something you wish to pursue)
How do we know what to hunt for?
Each member will have a wishlist. A wishlist can contain up to 3 desired items. The list can be longer, but our main focus will be on completing the 3 main items. The way hunts will be determined will be based on leader decision. You can improve your odds by being friendly and attending hunts frequently.
The item I want costs way too many points... Does this mean I'll have to help others and wait til I have enough?
Not at all! Due to the fact that some weapons are super rare and have a limited number of spawns per level the chances of find the weapon are absolutely horrible. This is why the item in question is so many points. You can still be chosen to hunt this item even though you do not meet the point requirement. For this to happen though, you would have to be an active member. In the event the item drops and you do not have enough points, you simply receive "negative points." Negative points are not a bad thing, but members with positive points will most like have their wish list prioritized over yours.
Okay I'm going to stop myself there cause if I don't I bore everyone to death. There are obviously more factors to consider but that basically how I would set it up, but I think the core group of us probably take this game more as a casual approach. This is just the hardcore MMO me talking.