Ras,
An interesting idea for sure!
As BB mentioned, that is adding a lot of weight to some already heavily loaded floor joists in the attic.
Also I would be concerned about what would be growing in the water and how we would get it off the PM when we did drain them. And if any of the containers started to leak, the sheet rock below would be ruined.
I think we need an answer from a scientific test showing how much heat for how long has what kind of effect. PM may not have this data hoping that their toys self destruct over time so folks have to buy more. The concept is called obsolescence, and it is quite common.
It may be a very long wait for the magic answers, Pgal