I think we have a draw here....50/50 for boredom vs mineral deficient.
Im going to say that the mare likely is mineral deficient because she has been out on pasture with no grain or supplements her entire life and sadly she is likely lacking some things.
One way to be sure would be to have her tested, another is to simply take the OPs word that by adding the minerals to her diet she has stopped chewing. It is quite possible. Horses will seek out what they lack, as mother nature intended.
As she has also already posted on more than one occasion to some deaf eyes and ears on here, the mare is getting a huge amount of hay every day so she is not lacking roughage. She may become a sherman tank tho LOL.
Keep doing what you are doing OP. Whatever works and whatever keeps your lady happy.
Chewing wood can be from boredom, maybe they are playing dentist, or because a horse has ulcers, or just because they feel like today is the day to take the bark off that tree. No need for anyone to worry about it too much. If you find a way to stop it that's just great. Generally speaking its not a problem in any case.