The Guidance sure doesn't discuss using a third party to do your due diligence for you. I would think if you could prove/document why using that third party to "vet" your appraisal mangement company would benefit your insititution much better than if it was done in-house I could see going that route. It's just common sense, as long as you can document, document, document enough to convince your regulator why going that route enlists expertise not available in your institution.
Full disclosure - I'm in to the "common sense" approach with this guidance. This is based on the many conversations and interactions I have had with our OCC examiners over the last few years. Others might look to the absolute letter of the guidance not addressing this specific area and disagree with me.
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Just working here until I get my letter from Hogwarts.