Old topic, but I have valuable advice to contribute regarding the Brigand's Chart quest for those still interested.
I did a lot of testing with it last year, and with the help of others' input, discovered that there is only one mob you can fish up during the event: Puffer Pugil. If you fish the pugil up during the event, instead of having someone else kill it, have someone HOLD it. By doing this, you will only fish up chests.
The key to this is to A. Get a brigand's chart on yourself and one or two others characters in your party that will be helping you, B. Stocking up on as many marlin as you can hold, and C. Setting aside a chunk of time to do these quests back-to-back.
Odds are, if you fish up the pugil, you won't have enough time to get to a 4th chest. By holding the pugil between attempts, though, all subsequent attempts will yield only chests. So, after you use your first chart, hopefully you fish up the pugil, and you have someone hold it. Then, have the next person use their chart, then the next person after that. Once all your charts are gone, start warping to Selbina to turn in MORE marlin to get more charts.
By the end of the day, you will have fished up more than a few 4th and 5th chests in the event (those the are able to yield penguin rings), and hopefully have found a few rings. One word of caution, though: The puffer pugil slowly gains in strength every 10-15 minutes (if I recall correctly; the interval may be shorter). It's not by very much, but by having him out of water for a long time, he might become a nuisance. Luckily, by zoning the pugil (use the Bibiki Bay zone nearby), his strength will be automatically reset.
edit: Here were my results for the last time I did this.
112 marlins in inventory, plus 3 people (including myself) having Brigand's Charts:
1st chart: 4 items, no mob
2nd chart: 3 items, 1 pugil (Held the pugil from this point onward)
3rd chart: ring (4th chest)
Then I kept warping back to Selbina to turn in more marlin for more charts...
Charts 4, 5, 6, and 7 all dropped rings as well (all 4th chest).
I ended up keeping three of them, and giving two to one of the fellows who helped me.