We have one of those rubs that you hang across the barn door or wherever. The Longhorns loved it and and they were always rubbing on it, but they were big on rubbing trees, barns, each other... The calves I have now don't like it and seem afraid to go under it, even though I put their grain inside to where they have to go under to get to it. It is like watching then do the limbo. I hung it pretty low because they are short and I wanted them to get the full benefit.
To answer your question, I don't know of any organic product that would stick for long, especially if they were rained on. We sprayed ours for flies in addition to the blocks we have out. We also sprayed their barn down real well and cleaned out the litter. Flies are bad this year, I noticed. I bet there are organic products to put on the rubs, but mine don't use it much, so they wouldn't get much benefit out of it.
There are surely websites advertising stuff like that.