How to get USB-mouse output from capacitive touch panel?
I don't know what kind of data comes out of that FPC cable, or even what its pinout is. Is there a well-known standard?

no, not that I'd know of.
You'll need a datasheet, or extensive time and knowledge to reverse engineer this.
Then you can write some controller – maybe that can be based on a microcontroller, maybe you need special analog signal processing, depends on what your touch panel actually outputs – to get some digital reading.
Then you can add a USB controller to make this a USB touch pad. Can't make it a USB mouse – a touch screen gives absolute positions, a mouse gives movement, different kind of information.