I wouldn't want to put a horse on one of the oral paste calmers for being ridden by customers. It just seems to me the horse should be properly schooled and retrained before going out on trails, especially as its an OTTB- the instinct to gallop will still be there beneth the calmer. If the horse is too spicy without the calmer to ride safely, perhaps the owner should ride him and retrain him until the horse is comfortable being ridden out.
I'm not against calmers generally- in fact more than once I've wished I had a calmer to hand, just I would expect a riding school horse to be sane enough to be ridden without being calmed chemically. Perhaps using a mild calmer in the feed, like magnesium based suplements, would be a more long term solution. The horse should feel less on edge all the time so should learn to chillax a bit more!
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz