Hmm, I've been messing with this for a bit, can't seem to get it to work either.
Here's some old code that used to do the same thing (but no longer works):
if (ClassTrainerFrame_Show) then
local old_ClassTrainerFrame_Show = ClassTrainerFrame_Show;
function ClassTrainerFrame_Show() old_ClassTrainerFrame_Show()
Since the commands are tied to the Blizzard_TrainerUI loadondemand mod...I'm not sure if this is possible anymore in this format.