if you're caught, they might ban the account. I doubt they'd ban the system though considering what you're trying to do can be done on a nonjtag/rgh. I was unlocking achievements a long time ago with my flashed drive. I was unlocking them with an app. I did get caught. all microsoft did was reset my gamer score. neither the account nor system were banned. I never really got that much game time on the 360. however, what might happen is speculation. personally, I wouldn't use it you have a lot of purchases.
edit: since I don't play my 360 much, or really anymore, I changed my account name. that would be a red flag, I guess. I would never use that account on a retail system either.