Concerning the use of other extensions through the WMP, I have found very interesting stuff in this thread :

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1248889032

The idea seem to come from here : http://obex.parallax.com/objects/471/

Here are the key points :

Instead of initializing by writing 0x04 at 0x04A600FE, you write 0x05 to use WMP + NC. (Maybe wait a few miliseconds here). Then you write 0x55 to 0x04A600F0. The contributor suggests that this is for nunchuck and that their is an other mode with 0x07 to enable a Controller Classic.

The data returned in the extension bits are alternatively for the WMP and the other extension.

3 bits seem to be dedicated to bookkeeping :

- - Byte 5 bit 1, which is 1 when the report is for the WMP, 0 when it is for the NC
- - Byte 5 bit 0, which i don't know what it does but is 0
- - Byte 4 bit 0, which seems to be always 1 when the wmp is connected

As the original data mapping for the Nunchuck already uses these bits, the original contributor proposes this mapping for the NC data in this mode :

	7	6	5	4	3	2	1	0
0	SX<7:0>
1	SY<7:0>
2	AX<9:2>
3	AY<9:2>
4	AZ<9:3>							1
5	AZ<2:1>		AY<1>	AX<1>	BC	BZ	0	0

As you can see, in passthrought mode, the accelerometers values of the nunchuck lose their least significant bit.

Their is also a Controler Classic mode that reorganizes the data otherwise, see the original code for that.

With some little hacky tests, I got this working with a nunchuck, getting alternatively the WMP report and the NC report. Some code is available in the svn.

I get this EXT_ID when a nunchuck is connected to the WMP and I initialize both : 0xa4200505