Looking good Matthew 🙂
I think to minimise the risk of damage you should supply it with a wire already soldered to the board for the audio then if the user wants to inject the audio they can strip the other end and wire it to the DIN socket themselves.
Have you considered just leaving pads available on the DVI PCB header board so we could wire our own 3.5 mm jack to the PCB:
Whilst not confident enough to soldier a wire directly to the F18A board (see first quoted post above) I suspect the DVI PCB board would be easier!
It would mean drilling another hole although it would have the advantage of it being entirely up to the user where they mount it. Of course you could just bypass the F18A and DVI PCB header and wire the audio jack straight to the DIN socket anyway.