Interesting ... never heard of that before.
It can be done and it doesn't have to be "just the root tips". Way back when I was in college, there were a couple vandas in one of the public greenhouse on campus. The vandas were suspended roughly 4 feet (approx 1.3m) above a koi pond. The roots had grown down into the pond and were growing along the pond's bottom. (So something in the neighborhood of 4-5 feet of roots were completely submerged underwater.) The plants were happy, the submerged roots were happy (complete with green growing root tips), and the koi seemed happy.