The only reason those pads say not to use on plastic is because they don't know what thermostats are. It is very true that you should never use a UTH or heat tape on a plastic box if it's just plugged into the wall uncontrolled, which is how they imagine the use of their pads. If you have a good thermostat to control the heat, you can use ANY heat source on a tub...even flexwatt.

The best way to use a zoomed pad on a tub is to tape it there with foil tape. DO NOT use the adhesive on the pad, as it's very hard to remove once placed.