I'm not necessarily sure that it would be something that can't be done.
As far as I remember from looking at the code, and from working on other things, the Bank Container works as a Layer of the Mobile.
So, should you move around that Item from one chars Layer to anothers whenever they call the bank, should be relatively the same exact thing as your doing now. Its only affecting the one logged in character at a time.
Unless you are trying to have a normal bank and a shared bank.
In that case, I think the best way to do it would be duplicating the shared bank and creating a new layer the same way it works now, and just move it from player to player OnLogin.
Obviously there's a lot more that needs to be modified from the original BankContainer, but might help you out there. If servers can handle thousands of players at a time's banks at once, I don't see why it cant handle Shared Banks.
Anyways, I know this is a recruitment page, so I'll delete the post in a day or two.
