I think the main problem is that we already have a lot of gold around for everyone, mainly in Dwilight, where everyone is Lord. In some realms, like in Sirion, we have our own way to distribute gold and I can just imagine the tons of gold some old people have with them. I know because I'm used to donate a lot of it, send it to my family or just keep it around. It's usual to keep 1/2k gold in bonds with my rich characters.
However, I can imagine a situation where a Priest of a poor religion try a loan to build a temple or something like this. Or Adventurers... maybe poor Lords that wants to build up in their regions fastest than he wants to wait for a certain number of taxes. In the end, I think the continent you choose will determine much of the success or failure of this idea. Mainly because few people will care to help, in a way or another, an enemy.
A banking system might just solve the problem of excessive wealth by generating massive debts. A few impatient lords borrow money for a new project. Now they owe the guild. They're making payments on that loan and they can't give their knights extra gold for their units, so those knights borrow from the guild. Before you know it gold is a more valuable commodity then it used to be.
