I don't think it's at all ridiculous. I think any disease, including Lupus, should be open to all possible therapies. I definately flare up when I get stressed or over-emotional. I'm taking an anti-anxiety to ease my anxiety, but I will be supplementing this with meditation and Tai Chi.
The mind is a very powerful tool for healing. If you believe in the hypnotherapy, I don't see any reason why it shouldn't be included in your health regimen.