I know that you all want tools to improve guild cooperation. I really believe that a scripted guild treasury is not in the best interests for the game by the time we already have national treasuries and Banks.
After all, Banks right now, can facilitate these needs, indirectly. Remember, Guilds are 1-man-control whereas Nations are not, which should make the management of such a concept easier. It is similar if we added a guild treasury, as it could be managed by 1 person alone- the guildmaster. It should make no real difference if a Bankers does this instead.
You can even charge your members a daily fee, to be used in times of need, with the current system. There are two ways to achieve this convincingly; either create a guild Bank (the guildmaster perhaps) or use a Banker you trust to do this for you. (Same nation guilds have extra options already, for a long time now)
To charge a fee, you can either tell everyone to send a montly or a weekly or a daily payment to the Guild Treasury manager (perhaps the guildmaster?) to his own Bank or to the Bank you are using (Bank levels of course matter here a lot). I realize that the limits are going to hinder things but these can be resolved in various ways (buried gold,nation treasure, gold bars, ships) that the Guild Treasury manager must find and use.
To help you more, I will tell you of another way.
Create loans for every member which they will have to pay on standing order. Loan payments do NOT have ANY restrictions. So, you can create an exact fee (given the option that they return the initial payment as they get it) that will be collected without the pain of any limits in mind. If someone stops paying, you throw him out or whatever.
So, there, you have your guild treasury already so I will signal this as Now possible.