I think that if you need to only index some folders, then you would have to create a link to those folders
inside the MPD music database instead than, as currently is, to the whole /mnt/USB device.
But, of course, as soon as you plug-in a USB drive with a different folder structure it would not work anymore
and you would have to re-create those links.
I don’t really know a smarter solution.
For what, instead, concerns your question on USB plug&play, I think you mean the fact that USB drives
are not “seen” by volumio until you reboot it. I also struggled with this problem, and for me (volumio 1.4 on
raspberry pi) it was due to the fact that udev service is not running.
By manually starting udev (sudo /etc/init.d/udev start) the problem is solved and the USB keys are correctly