Is it possible to add arbitrary folders to the media scanner?
If you are comfortable using the command line then you could use the directions in Can I trigger a media scan via the command line?

Pass file:///sdcard/usbStorage to the media scanner.

#!/system/bin/sh

export ANDROID_ASSETS=/system/app
export ANDROID_CACHE=/cache
export ANDROID_DATA=/data
export ASEC_MOUNTPOINT=/mnt/asec
export ANDROID_ROOT=/system
export BOOTCLASSPATH=/system/framework/core.jar:/system/framework/core-junit.jar:/system/framework/bouncycastle.jar:/system/framework/ext.jar:/system/framework/framework.jar:/system/framework/android.policy.jar:/system/framework/services.jar:/system/framework/apache-xml.jar:/system/framework/filterfw.jar
export EXTERNAL_STORAGE=/mnt/sdcard
export LD_LIBRARY_PATH=/vendor/lib:/system/lib

am broadcast -a android.intent.action.MEDIA_MOUNTED -d file:///sdcard/usbStorage


I can't test this for you so let us know if it works.