One problem with the built in serial component is that it does not include d2xx.jar needed for communication with FTDI usb to serial converter. If you connect this device you get an error “Java Lang.NoClassDefFoundError: Failed resolution of Lcom/ftdi/j2xx/D2xxManager”. I guess this is from the PhysicaloidLibrary. It is the same problem with the com.pavitra.5.aix that make use of the PhysicaloidLibrary.
If this is solved, then some of the Arduino clones can be used with App Inventor.
A better solution can bee to use https://github.com/mik3y/usb-serial-for-android. This library seems to allow communication with most of usb-to-serial chips used for Aurduino clones and usb to serial converters.