I believe that the question about robbing gold from anyone who wasn't already imprisoned was something that had been rejected in the past several times. Even if it is the banker, giving infiltrators that option will only create more problems than it would solve. For one, then that means the realm itself might start having some greedy infiltrators trying to get lots of gold without being detected from their own banker. As well, that could make the position of banker even more undesirable.
I don't really care much about shifting towards an action banker style for the position, as people would no doubt understand what they're getting into when running for the position. If it's appointed, well, hopefully the ruler knows what he's doing. However, there will be complaints and I am quite confident in predicting that there will be at least one realm that makes a ruckus about how the banker position would become a gold-transfer mule position.
As for obtaining gold, in any realm that's not in its infancy, or undergoing active repair, the banker could potentially obtain a really large amount in short order by altering realm shares, doing some covert stuff, and whatever else. It's probably not that hard, and I do believe there's even a "tradition" of sorts of shrewd bankers swimming in gold in almost every established realm. In short, for bankers of realms worth anything, gold is by no means hard to come by. Whether that means they can really finance an entire army is another question.
I think that while this option has the potential to bring something back to the banker position that was lost when realmwide food moving was taken away from it, there has to be some sort of restriction on just how often and how many nobles the banker can handle. Something like the militia assigning by generals could be used as a model. Don't let the banker keep handing it out all the time, or to everyone. Maybe give one or two, and then force him to recharge or something.
