ZooMed recommends using a thermostat or rheostat with their UTHs already--they do know what one is.

As I said, there are UTHs made for use with plastic and acrylic cages--it's not hard to find one that is safe.

If you use a UTH made for glass only with a plastic cage, and a thermostat, it will PROBABLY be ok. But you need to make sure that people understand, when you are telling them to DO this, that it is at their own risk, and it's against manufacturer recommendations. If their animal is killed or their house burns down, it will be their own responsibility for misusing the product.