What the placebo effect may be, instead, is simply an emotional boost that makes the person have better spirits, mostly because she thinks that she's getting effective treatment. There may be no connection between this feeling and the health outcome.
There are a few things that come to mind that might be relevant here. One, thoughts can affect us physiologically as you've said. Think of a naked woman. The intermediary between brain and body is a chemical. Fear releases chemicals, guilt releases chemicals, many emotions are linked to chemical mechanisms. The endocrine system is a chemical communication system in our body. We don't know the entire language. The regulation of a massive number of biological functions is done chemically. Blood pressure, electrolyte balance, sugar intake, and even our growth from birth to maturity.
A simple meditative exercise can lower blood pressure and cortisol. Cortisol suppresses your immune system, so meditating can boost your health. I'm sure there are countless other ways in which thoughts can affect physiology via chemicals.
The immune system is like the US military. Specialized pieces with different areas and sizes of focus. But composed of individuals. When at war with some diseases, it's nearly a stalemate, but one side is slowly gaining ground on the other. If we increase the rate that our immune system produces soldiers, the tables will turn and ground will be recovered.
Another thing to consider is belief. Belief itself isn't remarkable, but it is required for some physiological responses. You will feel fear if you believe someone is serious about killing you. But you feel no fear if you know them well and realize they are joking. So belief is a primary variable in some cases. Without it, no physiological response. With it, there is a response.
Think of a woman who is fighting for her life against cancer. Everything is going downhill until someone gives her a magical pill to cure cancer. It's a sugar pill but she thinks it's medicine. What would happen insider her head at that point, and in the hours following? Daydreaming about spending time with grandchildren rather than worrying about never seeing them again. Even if the thought is fleeting, it could alter your chemistry. Just like the flicker of an arousing thought can alter your chemistry. If it is cortisol that is reduced with that thought(a similar reaction to meditation), the effect could be cumulative. With less stress, the woman's belief in the pill would be strengthened, and she'd have more stress-reducing thoughts(or less stress-causing thoughts). With the increased effectiveness of the immune system after cortisol is suppressed, the difference could be enough to win a war against invading disease. A war that wouldn't be won without the reduction of cortisol. You'd be cured of whatever that specific disease was, simply by believing you were. This isn't the magic of belief, it's cause and effect, but very complex.
So I think placebo can effect a cure, but reality is far more complex and there are likely a ton of regulatory chemicals on both sides that would need to be considered.