Not sure at all about your herpstat...but as far as being safe to apply heat to plastic...

Here is a chart with the most common plastics used in today's industry, along with their melting points........all of them being above 250 degrees. I would like to see a uth get that hot, even uncontrolled

So you'd be fine as far as that goes....my question about this type of cage, is do people mount them inside? Or on the bottom?? They seem kind of thick, I wonder if the heat would radiate very good all the way through to where the snake needs the heat to be?