I want to add a shower in my large half bathroom, my problem is it's on a post-tension slab. Can I slowly chip away and expose the cable without much trouble, or is there another way.

You have to x-ray those types of slabs to find out where its safe to cut, you don't simply chip away at conrcete to find the cables . If you are in a condominium or a highrise... you have to do this in conjunction with building managements and their engineers approval / direction.

Is there any way that you can get to the drain and water supplies without going into the floor? Maybe put a false floor on top of the slab?