In fact, if you've flashed your GBAMP with the 2.11 chishm's firmware hack, you already have the secret feature :-)
Of course, it is removed in the source code, but it was in the loadARM7 routine.
Basically, it calculates something with your MAC adress, and with something written somewhere inside your NDS ;-)
Maybe Chishm gave to some people a tool to calculate the thing you had to write into your NDS. As I don't have this tool, I made a bruteforce calculator, and tested it.
It worked with Yoshi. In fact, I only test yoshi, because I'm not really interested into playing, only hacking :-D
Liranuna, am I right?