I have to agree for the most part with this suggestion. I can see where they would be problems (which you have mostly addressed already). But it might bring a whole new meaning of cutthroat to these forums.
Members might make it their entire goal to try and get in with the biggest FG bank guild and work their way up. Then when they get access or whatever they could just scam and take the FG. Since d2jsp has a strong policy on not giving FG back after it has been scammed (which I have been through personally) the guild would be losing out on their hard earned FG. It would seem like it would be better to just keep the FG for ourselves rather than risk it all in a bank.
Like you said, you were trying to implement some sort of a personal loan system in between yourself and members. Sure it's more work for you and the other members that loan FG but IMO, it's probably the safest way until all the safety concerns can be fully addressed.
Just my thoughts.