It worked it worked it worked!!
I did some easy groundwork stuff today, and at one point just sat down in the field and let her figure out for herself what to do. I had food in my bag and the clicker in my hand, so she stayed with me, trying out various things to get rewarded. Then she did something she likes to do occasionally, stick out her tongue. Click and reward. She shook her head at one point - click, reward. Then came the big thing: she opened her mouth - click/reward. Then again. I added the command (open mouth) and pretty soon she sort of figured it out, with me sitting on the ground. It became significantly more difficult when I stood up for some reason though, haha.
But the point is: she has suffered some kind of major trauma to her mouth. Getting her to open it was drama, she refused to, no matter what you did. Even the dentist needed 20 minutes to accomplish it, he said he'd never ever seen a horse so stubbornly keeping her mouth closed.
AND NOW SHE OPENED HER MOUTH VOLUNTARILY FOR ME!
All because I rewarded her for her own initiatives.