without knowing your sub, it's a bit hard to know exactly which buttons would make her feel humiliated. Reading her posts, I would think that taking control away from her would be a difficult thing to do, since she describes herself as a 'stubborn hard headed smartass'. It would depend on one main thing - is her wish to be submissive to you greater than her need not to be 'found out', especially by her family? If so, you have a hold over her there that can be slowly made more prominent.
Don't get me wrong - there are many things we do that do not involve any kind of humiliation, but when we started experimenting with our dom/sub sides a few years ago, it was hard to remind her that she was the one who started this. Neither of us had ever been with a woman before; it snuck up on us! She was very used to being decisive and in control, and since I had worked for her for a while, that carried over into our friendship. It wasn't until I left my job to become self-employed that we began to move into a new pattern, and her fear of public exposure was a tool for me to use.
There are other means of public humiliation that you can use. One of the ones I use on her: In one of her pairs of pants (trousers), I opened up the lining of the pocket. It's such a handy way to quickly get my hand on her pussy. If we're in an elevator, or an empty hallway, taking a walk in a secluded area of a park, or even in a big crowd of people, where you can rub up against her back. <sighes and drifts off into a daydream> Where was I?
Oh yeah. Anywhere that you can stand close behind her, without it being too obvious, you can slide your hand into the open pocket, and around to her pussy. It should be easier for you, because it wouldn't be odd for a woman to be sitting or standing in front of a man, leaning back on his chest. If she is wearing a long tunic top with a slit in the side, someone would have to look really closely to see where your hand is. So, go find yourself a slow, unoccupied elevator, stand behind her, and go to it! You never know when the door might open onto watching eyes ...

Want more? I got a million ideas! As you may have noticed, I am a risk taker ... she is not!