To put a heat source on the ground in a reptile enclosure and maintain the same safety standards compared to flexwatt outside the cage, it would have to be completely waterproof (not water resistant) I should be able to put it under water, snakes get things wet. The cord leaving it should be heavy duty, can't have conductor exposed obviously, then you need a thermostat with a temperature probe to be on the heat rock itself, secured in a way the snake isn't going to move it, also making sure the cord on it will not be compromised in anyway. So now your at near the same "safeness" as flexwatt outside of the tub of a typical setup and now the snake has heating spot the size of the heat rock., instead of one wall or part of the floor.
Even after all that outside the tub is safer and more ideal.