I've (briefly) flown RW (and done some LL in the mighty 'Dog), live 5 miles form Benson and have ridden for over 40 years.
Sven is right about blade slap - the sudden increase in volume and pitch will spook a horse.
Hi viz needs to be on the horse, and on the rider. The rider's hi-viz is aligned vertically, while the horse's is more horizontal - the two need to be combined.
However, in summer even a hedge of 10' will obscure a horse from a LL aircraft until quite late, and many lanes and tracks have foliage much higher than 10', and right now it is very thick. Riders on moors/heaths will stand out, but in leafy Oxfordshire it's easy to hide a horse.
This picture is taken from ground level:
and so it's quite un-representative of what the pilot will see. Note that the best bit is the hi-viz hat cover. It's the highest point, and it's visible from 360 degrees. If all horse riders used one it would be a good start.