we got 15 keets and though 5 of them mysteriously disappeared (we fixed the problem, which was escape + cats) we put them under one of our ever broody buff orps.
she hadn't been broody for very long, which must have been quite a surprise. plus, the guinea babies are really super active. i bet mama hen is questioning her broodiness decision and her unusual offspring right now.
thought anybody else trying this might like to know: we put them in a cage with a nestbox under the buff orp but she didn't like them at first. when she "roosted" at night, dh snuck in and put her back on the nest box and the next morning, she had apparently forgotten that they appeared out of nowhere, and helped them eat and stay warm. hooray!!