I've used extensions in the past which had set screws to keep everything together. I don't know where to buy them, but I would recommend looking for something like that. Otherwise there's a good chance you will lose something in the wall.
I'm definitely on the "don't do residential" side of the fence though. Residential cabling requires additional tools and skills that you don't need for commercial cabling. If you don't plan on doing a lot of residential, it's not worth doing any in my opinion.
Something tells me this picky client won't be happy no matter what you do. He won't be happy if you damage the wall, but he probably won't like the bill he gets after you spend 4 hours trying to pull the cable without cutting into the wall. And what if you spend lots of extra time trying to do it his way and then you still end up needing to patch the wall?
Use caution with this one.
