It's not possible to do this in the current version, i'm afraid. Even in software mode, you would have to deal with the zTrick jPCT can use. I don't think that i'll add a getZBuffer() method to FrameBuffer, but i can imagine to change the int[] that the zBuffer is protected. That way, you could access it, but only by extending FrameBuffer. That makes it clearer, that it's actually a hacky thing to do.
Would that help?