Very nice! Looking forward to seeing it.
I've never tried drilling, but i did use a hammer and chisel on my rock....very very easy to break a whole piece in half and was insanely soft. I would think, depending on the size of each rock, that a hole large enough for PVC will crack the rock. You might have better luck with something like base rock that has yet to become full live rock. Wish i could help more. I've seen it done on RC and it looked great.