The cables indicate they should not be used on drain pipe (supply only). I expect this is due to a fire hazard if the drain is empty and the heater is running? Comments? Someone with more knowledge than the 'plumber' at the HoPo?
From their webpage: Warning: This cable has been designed for the sole purpose of preventing metal and rigid plastic water filled lines from freezing. DO NOT use this product on fuel lines, drain lines, hoses or buried pipes.
Guess that's right.
Another one to scratch from the list.
That leaves three options as far as I can see (if the insulation doesn't work).
1. Try your light bulb method. 2. Move the line away from the wall as stated above. 3. rig something to drain the top foot of water or so from the pipe.