Actually, this is not the first time this question has been posed to me. Like Bernie, I’m a big believer of the benefits to be gained from working over these type of exercises for nearly every horse and rider. There are almost an infinite variety of ways to use them for improving both your and your horse’s balance, rhythm, awareness of stride and impulsion, and focus/concentration – all without the physical stresses that come with a lot of jumping. In fact, I believe that this sort of work is something that is hard to ‘overdo!’
That said, I understand your trainer’s rule about schooling without supervision. While I am sure companies offering insurance coverage for professional trainers are likely to vary in their actual wording, I would guess that few trainers are willing to open what might become a ‘can of worms’ that could result when students and horses under their care, custody, and control are permitted to set ‘obstacles’ (no matter how tiny) to work on their own. They might trust you to limit your work to the sort that Bernie introduced you to, but then find that others under their care took it beyond that and got themselves into trouble. With risk of injury, most responsible trainers take the ‘better safe than sorry route.’
Unless you have the ability to keep you horse at home, where you would have the freedom to do this work with raised poles, I’d suggest sitting down with your trainer and get her blessing to work over simple rails on the ground on your own. She might not worry about this getting out of control as there is no discussion about what constitutes a ‘raised pole’ versus a ‘jump.’
While not as effective at getting the horse’s attention as raised ones, ground poles still offer lots of benefits. Learn to feel and mitigate any small changes in your horse’s rhythm (as well as your own perfectly centered balance) as his stride adjusts as necessary to accommodate the rails. Both of you will benefit. If your horse starts out seemingly without much respect for them, persevere and he will discover that improving his attention and balance is far more rewarding than continuing to ‘stub his toes’ at those pesky poles!