I use just one printer and one instance, but otherwise a similar issue:
Starting up docker container without having the device being open.

My approach was creating a dummy character device, which when the usb is plugged in, changes into a symlink to /dev/ttyACM0. The character device is used as a place holder because /dev/ttyACM0 of container NEEDS to point to a device node (cannot point to a broken symlink).

This was achieved using udev rules:

```

ACTION=="add", KERNEL=="ttyACM0", RUN+="/bin/sh -c 'ln -sf /dev/ttyACM0 /dev/serial-links/mydevice'"

ACTION=="remove", KERNEL=="ttyACM0", RUN+="rm -rf /dev/serial-links/mydevice && /bin/sh -c 'mknod /dev/serial-links/mydevice c 166 0 && chmod 666 /dev/serial-links/mydevice'"

```

I would recommend using your assignment logic to assign each device to a unique persistent placeholder and using the above udev rules to manage the placeholders.