Hi @wardaddy ,
as you said, maybe connect to the 40-pin header UART would be the better way.
When we initially designed this switching logic, apart from the UARTport connected to the Bluetooth channel, the other UART port controls were switched together with the audio playback source. For example, the UART port on the 40-pin header is switched along with the I2S on the 40-pin header. This approach is beneficial for the control signals on the playback source side. Your current idea slightly deviates from our original design, but I can modify the CPLD firmware to accommodate your current solution.
Before doing this, you may want to first familiarize yourself with the CPLD firmware upgrade procedure. The CPLD upgrade process for the Tone2 and Tone2 Pro is similar, with only minor differences in the connection.
Tone2 Pro CPLD Firmware Upgrade Guide - Audio - Khadas Community
The UART port on the 40-pin header also uses the same configuration (3.3V logic, 115200 baud rate, 8N1, no flow control), so you can use it with confidence.
Regards,
kenny