This is not a bad idea at all. I have been thinking of doing the same. My fences are all electric rope, so my horses don't have anywhere to scratch, except on my sheds and round pen
Someone around here was advertising 4' sweeper brooms that fit over a 4" fence post that the horses could use for scratching. I've seen these for cattle, but don't know if they would cause problems for horses or not with the bristles coming out. I have also seen the scratching pads you mentioned and they look safe and effective. They also look like they won't tear out mane and tail when the horses scratch.
If anything, I would make your scratching posts fairly tall, as most horses like to stretch up to scratch. Also, use some secure hardware (screws instead of nails) to attach the scratching pad so the fasteners cannot work themselves out.
Good luck! Let us know how this works out for you.