All ideas can and should be tried. I'm a fan of simplicity, and it seems to me that the simplest solution is to have the client not show a note at all (in a reply) unless the note paid 1 sat to the OP. Its nothing. It can be lowered to a smaller unit when we get to smaller units. Some spam will still get through, if they pay, but most will stop when they math the total cost of an attack. I'd like to try this solution.