Hi Maurits,
You already found most of the solution yourself, great job! Unfortunately, the only way is to loop the output back into the CueCore2. This can be using DMX, but also using sACN or ArtNet if you have a device that can send it back.,
In your setup, the trick is to switch the DMX outputs. On the settings page, you change port A to input. Then set port B to "Universe A". You will have to switch the physical connectors as well.
The snapshot will then be recorded into universe A instead of B.
There is an API command for selecting the track. In ShowControl there is currently no way of selecting the track.
The API command is "core-tr-select=<track index>" for UDP and "/core/tr/select" with an integer for OSC.
In Kiosc you can create a button for selecting a track with that, combined with another button for creating the snapshot.
Good luck!
Greetings,
Bootsy