I'm not sure if it's possible automatically within a mod. From what I can tell there is no place to store data for all accounts. Generally the data is stored within the Account name(WTF/account/<accountname>/savedvariables/ or WTF/account/<accountname>/server/charname/savedvariables/). There may be some way to do it, but I'm not familiar enough with how addons save/load data to say with confidence it can or cannot be done. I can give work arounds though to have it work. There are 2 possible ways to make it work that I can come up with if there is no way to have the addon save and access data across accounts.

Manually: Copy the characters data from one account to the other. I've done this to import addon settings from other accounts to my new account. This would have to be done all the time though to keep it updated. Of course teh addons that allow you to see every toons bank/bag probably store the data in the accountname/savedvar folder which means you most liekyl would have to copy paste part of the file into the others. This could cause issues if you don't get the formatting right(miss a { or something and your file is screwed).

Other: If you had an addon that stored your bag/bank data then you could make some method to send the data while in game from one character to another. Either by a synch command, or autmatically whenever the toon closes their bank it would send to other party members with the same addon the contents of your bank(maybe use the hidden addon channel). I'm not sure how sending bank/bag data would impact framerate(would be a lot of data to send), so maybe a synch button would be better. You hit the button and it requests the data from other party members with the same addon. I say a synch button as I don't know if it would be a good idea to spam all that data to synch your bag as it would be changing a lot most likely and if it did slow down it could effect gameplay if someone opened their bag in combat or some other time when you wouldn't want any hangs or framerate problems.

I'm sure someone who has more experience in addons could make something work and probably have a better grasp on methods to make it work.

I'm sure someone else will answer with more clarity.