What is the single most effective (available and legal) solution to
getting rid of scale below the water line in a toilet bowl?

Kilrock-K (that's in liquid form not the gel) is the stuff I have
found most effective. Remove as much water as possible, pour enough
Kilrock in to coat the affected surface, brush in lightly and make
sure the room is very well ventilated. Leave for 15 minutes, brush it
and repeat until sparkling clean

Mike