Hhhhmmmm - Well this is supposed to be a 'discussion' forum, so I'll be honest! Sounds a bit gimmicky to me. (Ducks).
'Mindfulness' is used within cognitive-behaviour therapies because humans are very bad at staying in the present moment. We are pulled back and forth between the past (leading to emotions like depression, guilt, regret, anger) and the future (leading to worry/anxiety).
As far as I know animals don't have this problem. They live in the present all the time. Which is why they can be freaked out by a train thundering past their field and 10 secs later be grazing again.
Pain/fear does cause trauma but something in the PRESENT needs to trigger it off. Eg a raised hand, triggering off fear of a beating. This is unlike us humans who can stress out about anything just by remembering or thinking about it. So I don't see how teaching a horse to be 'present' is necessary.
Teaching a handler mindfulness, on the other hand, is likely to be very helpful as horses are exquisitely sensitive to the emotions of those around them. So calmness and prescence in the handler, will result in a calm, chilled out ponio.
I once had a perfect example of this at a trekking centre: there was a gallop along a wide track with a large stone on one side of the track near the end. Some trek leaders warned riders about the stone, and some didn't. On treks where riders were warned, many horses shied at the stone. When not warned the SAME horses ignored the stone!
If you find it helpful, go for it, but I suspect the effectiveness of it will be due to different processes than teaching a horse to stay present.