Thanks!
Alright, I think this portable is officially completed. I added a low-battery indicator based on daftmike's:
viewtopic.php?f=7&t=255
(the second revision)
I redesigned it so that it only needs two transistors, but does not have a green LED, just turns on a red one when the battery is low. Here is the circuit I built:
I also added a jack for an external battery pack/power source. It still cannot charge while playing, however, because the battery is only hooked to the charge port when the power switch is in the off position.
Here is the low-battery indicator in the case:
External battery port:
I did not drill a hole for the LED but instead stuck it under the screen, behind the plastic. When it lights up, it shines through the paint.
Just to prove the cartridge works:
External power supply plugged in: (the case isn't closed in this picture)