Can an AC condenser be wired inside the wall, underneath siding?
You could in theory do it inside of the wall, but the "device box" would have to a a Flush Mount 3R box, and that's not something I have ever seen for a simple AC disconnect. In other words you can't just exit the wall through a hole in the siding and run into the bottom of a surface mount box, and for all of the surface mount ones I have seen, you can't enter the box from the rear because that's where the disconnect device is. So although it COULD be done, I don't think you will find the hardware to make it happen (at least not to Code).