By all means, let them know. I'd love to see a ring of motorcycles around the cemetary. Those cowards wouldn't come near them. Go bikers!
I wonder if some truckers could lend a hand for a couple of hours? Parking their 18 wheelers to block out the #*$&&$@()&)!#$)!#$s would shield the family from their signs.
That's my thinking. Bikers won't tolerate them. No signs, no bullhorns, no obvious protest. Allow some dignity. Just seeing a biker honor guard lining the route should take care of business very nicely.