simply have some labels printed with the wording "Isolated - safe to
work as long as this notice is present" on one side. Print you advert
on the other, attach to a known live 11kV cable with the safe to work
notice outward and wait for trade to come rolling in.

Er - isn't this guaranteed to "kill" the trade?

OTOH - sell the labels directly directly to the copper thieves, and
tell them that so long as they attach the label first, they're
perfectly safe.