I just read through this whole thread which I found very interesting. Really liked the improvised fish pond. I've got 2 "tubs" about 2 metres long by about 1.5 wide that I've set up inground. They were part of an installation used by telephone companies, forgot now what the specific purpose was.
Spaced between and on either side of these are "mud ponds", made from plastic liners, for bog plants. Only problem is I set the "tubs" down too deep and have to lift them. It's part of the surface water drainage system that runs into my dam. But the water stands too deep during the wet season and I've lost a couple of plants to that. So it's still a work in progress.
But on the issue of overwintering tropical waterlilies, you should be able to get away with just storing them in dry sand. I have our native waterlilies which by virtue of location are tropicals. A lot of the waterholes the grow in (including my dam) dry out during the dry season. They keep their leaves until the ground dries completely. The leaves wither and the plants are then only tubers in the dry soil.
Our winters stay over 30C and the ground really bakes dry and hard. Soil (dry) is a good insulator so the temperature under would be fairly constant. In the wet season the plants produce small leaves once it floods above them. The leaves stay small and submerged while water levels fluctuate. Around the end of the wet season when waterlevels stay consistent or start to drop the leaves begin to grow up to the surface and they begin to flower. If they're in permanent water (the same species) they continue growing all dry season but die back a bit when the heavy rains of the wet season return.
From that it's most likely that most tropical waterlilies would die back to the tuber if they were allowed to dry out and then remain dormant if kept in dry (insulating) sand in a warm place. Worth a try if you really want to keep tropicals.
