Went trail riding yesterday with a friend. We were on the narrowest part of the trail, literally nowhere to go on either side,(straight up to the right, straight down to the left) when we heard motorcycles coming up from below. They stopped, thankfully, in time. Both of us were on green trail horses, and mine was blowing and snorting. The motorcyclists got off and tried to lean their cycles against the uphill side, which gave us an 18 inch path to pass right next to them.
I asked if they could take off their helmets and talk to us! Once they did my horse calmed down immediately and we very slowly walked by them. I could feel his heart beating against my calf, though, so I knew he was very nervous.
It was the worst possible place to meet them, but the three on the dirt bikes were so nice and careful. We thanked them profusely and went on our way. I've read others' accounts of bad experiences with motorcyclists on the trail, but these three were great.
A fellow rider turned his horse toward the noise of a motorcycle (on the trail illegally) and said those three magic words "Get that cow".
Horse took off in pursuit of the bike!! I have never been so impressed.
The author Mary TwelvePonies writes about getting her horse over his fear of motorcycles by having him chase them. A friend would start up the bike always riding away from the horse. If you have a biker friend maybe you could work with him. Maybe the biker could have carrots on him so when the horse "catches" him he gets a carrot.
Really, in my experience, the vast majority of bikers, motorcyclists, hikers, 4-wheelers etc do try, and want, to do the right thing and be courteous. It is the few 'rotten apples' in each user category (including I horse riders) that give the 'category' the bad reputation.
For all scary activities involving humans, absolutely engage those folks in conversation, you're quite right, once the horse figures out there's a real person in there, they get over their qualms quickly. I would add that speaking cheerfully to barking dogs in yards and such works, too. The horse will very much key off of your behavior. If you are fearful that the horse will get fearful, the horse picks up on that and you are not going to have a pleasant moment.
And cheerful education is always good- a hiker with a huge backpack is trying to do the right thing by 'hiding' off to one side of the trail, so just asking them to come out out and speak, maybe even pet the horse if they are willing, is a good thing.
When I meet up with a strange or noisy vehicle that my guys are nervous about, I will put my hand up and try to get them to stop. Most will. The only time I came across a motor bike on a trail and he decided to speed up by me instead of stopping, the horse swung around and let both feet fly. That guy will never know how close he came to getting kicked in the head. There's really no way to plan for what others will do on the trail. We can only pray and hold on.