I know you can get it using a TV converter (not tuner), but I was wondering if there was a way to get it strait off the board?
If your curious about my reasons check this cool link out:
http://pages.prodigy.net/tan.ax/GBAConsole/gbaconsole.htm (http://pages.prodigy.net/tan.ax/GBAConsole/gbaconsole.htm)
I don't see how that page explains your reasons, but that aside - if it was possible to get RGB from the GBA without using the add-on adaptors, why do you think everyone's using add-on adaptors?