Currently the usbinstall script requires the dosfstools extension to be loaded before it will let you proceed. From what I can tell, dosfstools is not needed if you are doing a EXT type usbinstall, so it might be helpful to detect if it is a EXT install and not require dosfstools in this case.
Changing line 252 to something like this should work:
[ $TYPE == "EXT" ] || [ `find "$TCEINSTALLED" -name "dosfstools*"` ] || MISSING="$MISSING dosfstools "