Currently, it impossible for a turtle to place a piston oriented the way I want (eg. turtle.placeDown doesn't place the piston facing up).

I'm using CC 1.78 and MC/Forge 1.8.9

No it isn't, you just need a surface to place against. Only when the piston is being placed in thin-air and needs to be placed against the turtle's face do you get the opposite orientation.