Note that pin 5 (Filtered Audio) and pin 4 (Channel Active) are both on the bottom edge of the connector. These pins are not accessible on the command board because they're underneath J502 and along the front edge of the board, but you might be able to tack a wire directly to the pad under these pins and route them back into the radio.
If you connect J502 pin 30 to J502 pin 31, the radio will power up without the control head present. All other signals should then be usable.
Pin 5 (Filtered Audio) is de-emphasized and muted, and the level is 60 mVAC per 1 kHz of deviation at 400 Hz tone, and 26 mVAC per 1 kHz of deviation at 1000 Hz tone.
Pin 13 (Detected Audio) is neither de-emphasized or muted. It's raw full-range audio of about 195 mVAC per 1 kHz of deviation at any frequency.
Many of the signals presented at J502 are routed to the two DB25F connectors on the front panel of remote-mount radios. The following image was extracted from the Basic Service Manual: