Does seem odd to pull chain plats for a survey. They are through deck type right? You can use a dye penetrate test, but as said before results are anything from conclusive. Any other testing is expensive. All commercial rigging is tested to a point above its rated working load, and retested at an interval. Never heard of this done on sailboat parts, could be interesting. Unless there is obviously something wrong, chain plates seem like they would be the last item of concern on the rig. The safety margin built into them is larger than any other rigging component.
My boat had a crack in the stainless cranze iron, surveyor just pulled me aside and told me to make sure the owner replaces that before I paid for the boat.