you don't necessarily need B9S, it works as long as the payload's sole purpose is to unbrick the MCU, and doesn't init any hardware other than eMMC and I2C (screeninit *will* fail)
but this is assuming that MCU_FIRM didn't corrupt so much you can't turn the 3DS on at all... as long as you can boot to the MCU reflasher code and MCU_FIRM's I2C code is intact enough then you can reflash it via software
...otherwise have fun with the testpads