Wow, I like the ivory soap idea. I'm going to try that one! I have used sunlight dish soap and chili pepper and I squirt it all over my barn. It worked for two of my horses, but not the current one. Still doesn't solve all the rest of the wood around though.
Chewing on green bark is healthy -- especially poplar / aspen trees. There are anti-oxidants and who knows what else in there that is good for them. If there aren't any in the paddock, can you get one from someone and drag it in the field for him? They won't chew on dry wood for nutrients. That's a myth.
Anyway, in my experience, their chewing (not cribbing), wasn't from boredom or bad teeth. I think it was from insecurity or a feeling of helplessness. Everytime I took a horse from the track, it would chew. This spring I ended up with just one horse here that hadn't chewed in over a year. I brought in a new OT horse and she chewed right away. Within a couple of days, the old one was chewing again too. I've got it just about beat again and I notice that every time I get it beat, the horse has settled down, become calmer and generally secure in the field. The more settled, the less chewing. I had a neighbour's dog come in the barn last fall that barked and made such a fuss that one horse had diarrhea that evening and wouldn't you know it, they both chewed that afternoon.
How long have you had this guy? Spend time with him and make him secure in his environment. Tell him no if you see him chewing and, yes, try the ivory soap. I know I'm going to. The other thing to think of is the age of your horse. Younger ones tend to check out things with their mouth more than older ones.
aaaand she only does it at certain times 
