how to generate frames using ffmpeg `select` filter with custom function

Question

I got a video, and want to generate some frames to be my video cover candidates which should satisfied with some conditions below:

1. frame in vary scenes.
2. frame contain person with good look, no eyes closed and etc.

I found ffmpeg select filter abilities, like

ffmpeg -i input.mp4 -vf "select=gt(scene\,0.4)" -frames:v 5 -vsync vfr frames-diff-%02d.png

can generate frames that have more than 40% scene change compared to the previous frames.

I just found that only built-in select expressions support and no customization support.

1. Does ffmpeg support external function/api/local command/script to give a frame select evaluation?
2. Is there any other solutions to complete my goal?

Thanks.