Im not too sure how hard this would be to code, but there is a similar system with the rented halloween weapons where they were only usable for a few hours before they expired.
My suggestion is to have any soul bound tool that is placed in the bank given a 24 hour lock after which it would become account bound until used again.
This way when new tools become released and someone would rather have the new tool on x instead of y they can move the previous tool to a new char and enjoy the new one on the other char.