Nooooo i need those coffers, becoming Sannin is one of my 'Things to do before I'm too old/busy/dead for S.L' because of the gems. Its annoying enough to get Sannin on a main, who the hell has the time on their hands to do it with an alt?
Anyway, I think I have a solution! Basically, whenever a person becomes sannin they should be UNABLE to use the coffers until they have had their [insert criteria that you use to check if someone is an alt] crossreferenced with other Sannin+ characters. If they dont match, then they should get the A-OKay, if they match with someone, they should be denied from using any and all gem coffers. (if your criteria or whatever works, technically it should be impossible for someone to match with anyone other than an alt.)
Or! solution 2: whenever a person becomes sannin they should be crossreferenced with other Sannin+ characters, if they match with anyone, their gems should be marked with some data or something that makes it impossible for the people who match to touch any gems deposited by that person, and the gems should stay marked, so you couldnt just run them though another char (eg: you use alt to deposit, your friend withdraws and deposits to remove the mark, then you withdraw)
I dont know if the second idea is possible but its worth a try, ANYTHING to get the precious precious gems back, accessing castles is a long awaited feature, it would be cruel to remove it now when im SO CLOSE!