It is an interesting idea. However, I think that guild taxes and maintenance would kill it. Any region lord could run up huge amounts of credit by depositing gold, then cranking up the regional tax rates to get it back for himself. Guildhouse maintenance would also slowly sap away your reserves, requiring people to continually put in money to maintain guildhouses, and thus generating bigger and bigger balances.
In addition, I think you'd also have a problem with the realms, if your judges start trying to enforce guild punishments at a realm level. What realm would be willing to allow that to happen? No way in hell I'd let you start banning/fining my nobles because they owe your guild money.
