Well, I broke down and purchased a chatpad and controller. Disassembly was a breeze with c0nsumer's photos. This morning I traced the J1 connector signals to the PIC 16F883. The good news is that the I/O connection is using the UART TX/RX lines.
The UART link appears to use 115200 8-N-1. There is a NULL character handshake sequence with some data exchange. I see this at connect and disconnect. I could be off on the link settings. Keep in mind that I am connected to a PC and not an XBOX, so the controller is not activating the chatpad. c0nsumer or someone else with a TTL serial analyzer should be able to sniff the link on an XBOX-connected controller. The J1 surface mount pins are large enough for micro test clip connections, so no soldering required.
Once we get the init sequence and decode the keycode protocol this will be a snap! I have two candidates for the UART-to-USB HID host side translation. I have included I/O information and partial BOM reference information below. I am guessing on U2, mainly from diode drop readings. I briefly reviewed the LED backlight circuitry. The U5 part has a "AUH" laser marking but I was unable to decipher the part. Not really an issue since I plan to interface at J1.
(Q3, Q5, Q6, Q7) PMBT2222A NPN Transistor SOT23 (W1P Marking):
(U2) BAV99DW Switching Diode Array SOT363 (JG Marking):
SMD Marking Decoders
http://lab.artematrix.org/papers/SMD_codes.pdf (PDF Version)