Several points

1: why? won't be as convenient or look as nice as a proper towel radiator.

2: I'm pretty sure the heater won't fit. I expect the threads would be fine
but on a towel rail the element screws in and goes straight along the bottom
rail. On a pressed steel radiator (single or double) there is a crank
between the valve fitting and the bottom water gallery.

3: cheapest engine or other mineral oil would be OK though (screwfix again).