Is there a good way to add a rebar-type concrete encased grounding electrode AFTER the slab is in?

Basically not without chiseling (or drilling and chiseling) in to find rebar (and it would have to be a 20 foot chunk unless it was all tied properly for use as a ground even though you forgot that part at the time.) Covered by 2" concrete is not actually inaccessible, particularly if the concrete is relatively fresh/green. You can do that, and patch where you chisel out, or you can drive a few extra ground rods (one more will technically do, but they are not that expensive...)

Note if you have (code acceptably, but practically dubious) buried your rod horizontally at 3 feet, do consider actually driving it and any friends you give it in. I have 5 driven from the bottom of a 3 foot trench (so they go down to 11 feet) myself. My concrete contractor ignored local code and did not provide a ground connection as is required in all new concrete foundations here.