Hi there, Realize this is an old topic but I figured Id add a bit to it anyway. We regularily use the Solo as you describe, by connecting directly to the Pixhawk through Serial2. The hack to enable is quite simple, just connect to the serial console of the Pixhawk on Ser5 and create a file /fs/microsd/uartD.en, then set APM parameter SERIAL2_BAUD to 57 and SERIAL2_PROTOCOL to 1.
It does, as mentioned, cause some interesting issues. Gimbal control breaks by enabling Serial 2 and this only works with firmware 2.3. Newer firmware seems to ignore the enabling of Serial2. If you have any idea why, please let me know.