My mare has a very thin spot from her blanket neck and my plan is to make falsies out of yarn. I already need to add extra yarn to that spot normally so this year it’s going to be more yarn than hair. As long as you match yarn to mane color no one can really tell. I particularly like chenille for this kind of fake spots since it really grabs onto the mane.
For a schooling show I’d just make the rest really tidy and carry on.