From an email posted to the main cavalier breeder's list. This well known show breeder asked the neurologist running the North Carolina study, Dr Olby, if dogs could be walked on collars (note: NORMAL collars, not choke, prong, or slip leads, which are far worse at putting pressure on the neck area). Her response:

We asked Dr. Olby about whether we should use harnesses exclusively for our
cavaliers. She said yes, in her opinion, that would be a wise thing to do.
That "just in case," the use of a harness might not "add insult to injury."
In other words, she thinks all cavaliers should be walked on harnesses all the time. This advice was not given for dogs *diagnosed* with SM -- she is advising this for ALL cavaliers because of the very high incidence of SM in the breed, and the chance that using even a regular collar for walks (setting aside jerks/leash corrections, which should obviously not be used at all for the same reasons) may exacerbate any existing SM condition.