well, my hack hasn't got a RTC implemented so I haven't really tried; I'd have to get familiar with coding with the RTC. I assume what you could do is once you start a bank account you could start a..."timer"? or whatever the RTC hack uses, then every time you go there it checks if the timer has been a week yet, and if it has then you could use special 0x41 from JPAN's hacked engine which divides variables and copy the bank account variable to another variable, use the special to divide it by 10, then add that variable to the bank account one. then you'd reset the timer. if it hasn't been a week, then you'd just keep the timer going.
im thinking interest could maybe be handled by a second person event to keep things from getting complicated. the player would then be responsible for collecting interest themselves.
idk.
couldn't this just be exploited and the player could just change the time on their pc and get unlimited money? just a thought