Stuff I couldn't find elsewhere:

To turn a device off, send: 00 00 08 C0 00 00 00 00 00 00 00 00

To enable wireless controller chatpad events, send: 00 00 0C 1B 00 00 00 00 00 00 00 00

You will have to send alternating 1E/1F keep-alive packets (every second or so, I guess) to keep the chatpad events flowing: 00 00 0C 1E/1F 00 00 00 00 00 00 00 00

After doing this, you'll start getting 00 02 00 F0 event packets (the 02 signifying chatpad data). The same data as the wired chatpad starts appearing at offset 0x18.

There's surely some way to enable/disable the LEDs, but I didn't bother looking.

See a full dump at http://brandonw.net/Xbox360ChatpadLog.zip for more.