well, the zorroII is there ONLY for providing a place for the ppc to connect to, no z2 cards will be used, and the amiga won't even know there is a z2 busboard present :-D
and putting the "good" power line on the g-rex is a bad idea, because the z2 powers the ppc card, which draws more power... anyway, I have installed it with the way I described and everything is OK (I asked a couple of friends on #amigahellas that have g-rex's first...)
EDIT: NOT by removing the plug indicated with an arrow, but with the aid of my cable... be carefull!