This Japanese guy, too, seems to be investingating firmware on atj213x series of chips:
Google Traduci
This British guy here seems to have a atj2135 firmware modify tool:
MP4 Nation Forums • View topic - mod tools for ATJ2137 coming later tonight
Also, help in english for the atj2137 tool, which seems a very similar chip (same atj213x family):
ATJ2137 firmware modify tools use help-TECHNICAL-Mp3 MP4 Player Firmware Downloads Base!