The MOSFET level shifter used in the Adafruit product is essentially the same as what we do on the current controller board, to shift the 5V I2C bus coming out of the Atmega 2560 to the 3.3V level used on the Depth/IMU board.
Over time we’re gradually moving away from 5V stuff- pretty much the only product using it anymore is the Arduino, and they seem to be working their way towards 3.3V as well, with the Arduino Zero (though with the current ownership issues, who knows what will happen…)
So unless there’s a whole lot of clamoring for a 5V product, I don’t see us introducing one.
The idea you have for the converter works, though you left out the 5V–>3.3V regulator that would be needed on a 5V board. This would probably be the most expensive part of the modification.
Please continue to post the work you’re doing, either on the forum or on the community hacks portion of the Dozuki site. The products we offer in the store are never going to be perfect for everyone, and we need more people to share how to modify things for their specific need. Thanks.