Skyjob is correct - I should have mentioned the magical new TALPA. Our advice for specially prepared runways is to downgrade the measured braking action one level, but that's it.
It's basically an ice surface that has been sanded, so I don't know what kind of contaminants you are allowed on it. For snow on Ice, or ice in general, TALPA prohibits landing for example. Am I allowed say 1mm of snow on top of a prepared runway? If so, what braking action should I plan for? Am I even legal to attempt a landing? Our books are silent on all these questions and would be interested if anyone has any answers.