udev rules can run a script, s you could have a scriot that is run when a device meets the basic requirements, USB and removable, that mounts each partition and checks for the specific file. Then it either passes the file to LO or unmounts the device.
Alternatively, if you hav control over the USB flash drives used, you can give a specific label to any filesystem containing the file you need and have udev check for that label.
Either way, be aware that udev blocks while it is running a script, so your script should detach.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)