I have a piece sticking out of my 10 gallon that is doing quite well. The plant is much happier there than it was in the pot (I bought it on sale from work because it was half dead).
Everyone I've ever asked has said do NOT put the leaves under water. Eventually they will begin to rot. A breeder in my group is big into plants as well as fish and has a huge pothos growing out of a 10 (maybe 20 long) fry tank. The roots are huge and so is the plant. I don't think he has bettas in that tank though... endlers I believe.