> 2017-06-16: playing a bit with udev scripts to automate actions upon USB stick plugin: I can do short actions (like mount/unmount) but no longer running actions from udev!
You could run the script you need from udev with "at -f /path/to/script now" so it won't block udev. Mighty need a slight delay (either "sleep $time" or something more intelligent to check a few times if the /dev/entry appears) in the script to allow for the drivers to load.
> lack of usb ports on PiZero
There's this https://www.tindie.com/products/mux/nanohub-tiny-usb-hub-for-hacking-projects/ made by a mutual acquaintance might help. I guess you'd want a 'real' usb port for the user to plug in the usb memory device, so you could probably desolder the usb plug form the zero, wedge a hub in between and then solder another hub port to the usb sound device