With house on slab, can exterior wall outlets be grounded to water pipes &/or ground wire attached to say a rebar pounded into grnd, after drilling through exterior wall, or is it required to ground through going into attic & then to meter box?

The outlets should be grounded by a green or bare wire that is included in the branch circuit wiring that runs from a panelboard to the outlets. If your trying to add a ground wire to 2 wire outlets, then all you can do is run a third wire from the outlets to a grounding electrode for your system. This may be your metallic city water line. If your grounding electrode is water line, then you can connect to the water line no farther than five feet away from the point of entry into the building.

If you use the rebar as a grounding electrode you may connect to it at any point.

What exactly are your trying to accomplish?