First of all, this "bank" would be more for helping out fellow guildies, than for making money. The charges would be applied only to benifit the guild. Everyone would benifit from the money gained from these charges though. This money gained could be used to sponsor competitons, build up adarah's stocks, contribute more to garions book supply etc...
Ok after stating that above, because everyone would have to feel like these charges are for the good of the guild,
I think that the best method for this bank to run smoothly would be:
Use of 1k incriments of money
Normal Peirod of time to have a loan for would be 1-3 weeks depending on the size of the loan and the use of the money.
Each person recieveing a loan would need a reason for why they were taking it out, as well as a plan for how they were going to use the money and be responsible for it when time came to pay up.
Storing money would require a one month bond, removing money before that time would cause a nullification on any interest gained.
I think that a sticky list on the forums would be best for people interested in using this service
I think that three people should decide on each loan, and each person getting a loan would be required to convince all three that they were responsible enough for it.
Everyone would be entitled to see the books at any time
(btw, Im an accounting major in my junior year, and I really enjoy this stuff, so this would be fun for me as well as good experience 
)
Variables that need to be discussed:
Percentages used on loans and bonds
Terms and Conditions to Loans
What to do in a case of nonpayment
Who to offer this service too
Compound Interest or Not on Bonds
So after all of this everyone should look at the pro's and con's of this idea:
Pro's:
The Ability to get quick cash to purchase large quanities of certain supplies to help train: such as leather, mana pots, thread, etc..
The Ability for the guild to gain gain more money, to help with what ever is needed at that time
For players who can not save up, being able to grab some cash, or store other cash some sort of a bond gaining a little interest
This would defintly support all types of players, from people needing efe's to others needing large quanities of wine to work on potions
Cons:
Well the biggest, this could flop and we could loose everything, Riva would be knocked of the entire map of EL (im just kidding there, but there is a chance having people go inactive, from time to time, or just skip out on payments, though I dont know anyone in Riva that would do that)
Some players may feel like they are being cheated, and could cause some conflict in the guild. This is the other negitive that can not happen, If I think that this will happen I will automatically trash the idea.
Other negitives would be, if this does actually work, and we do not have enough money to be able to loan out to certain people who need more.
As you can tell Ive put a little thought into this, and I think It would be really neat to try, because to my knowlege I dont think its ever been done in a game before.
If anyone has any intelligent input what-so-ever please tell me, b/c id like to see this work, but if you dont think it will speak up b/c Id hate to see this flop