When I first started collecting ball pythons, before I built my rack system, I had radiant heat panels screwed into the lids of two cages and a zoo-med UTH under the third. The most important aspect of this setup were the thermostats. As long as they are regulated and you check them frequently with a secondary measurement device such as a temp gun or a digital thermometer, I don't think you should have a problem doing it this way. A regulated heat source should never get hot enough to cause damage to a tub, particularly because the snake will suffer damage from the heat before the tub will.