Hi,
I examined the Nunchuk extension cable at the Nunchuk connector and found something interesting.
Pin 1 is bridged with solder to Pin 3.
Therefore connecting Pin 1 to Pin 3 WONT WORK for connecting the Nunchuk extensuion cable between the Motionplus Dongle and the Wiimote.
Sorry about the earlier misinformation on that point.
So, it seems the only way to get the MotionPlus Dongle working through an extension cable is to wire across pins 1, 2, 3, _, 5 & 6 individually, as it seems Pin 4 is not connected through at all.
If you are desperate for a circuit board to wire into the MotionPlus, the connector from the Nunchuk extension cable has all the pin pads available on a microboard, so its just a matter of piggy-backing on those and debridging Pin 1 and Pin 3 as well.
Thanks.
POST SCRIPT:
There is some interesting information in the Nintendo Wii patents found at the US Patent Office -
http://www.uspto.gov/Do a "quick search" for the relevent patent number.
For information on the connector technology of the Wii, search for this patent number - 7722409
For information on the MotionPlus technology of the Wii, search for this patent number - 7867089
Its good reading.