The iREB R6 for Windows is out there, its the tool that put your iDevice into Pwned DFU mode which makes possible to bypass iTunes errors 1600, 1601, 1604, etc, normally popup when user try to restore custom iOS firmware.
According to iH8Sn0w tweet:
“I updated the standalone iREB (r6) to support iOS 6 restores on the iPhone 3GS & A4 devices. Downloadable at ih8sn0w.com as usual.”
Download iREB R6 for Windows