It's definitely possible to hardcode this somewhere in the code. You could put a line like this into ArduPlane's setup function (or perhaps it would be better placed in radio.pde's init_rc_in function):
g.rc_8.function = RC_Channel_aux::k_flap;
That's certainly not the cleanest way to do it. The best way would be to set the defaults somehow through APM_Config.h but I don't think that's currently possible.