I'm afraid we haven't implemented that feature yet. However, it is possible to use donut series with colors containing an alpha of less than 1. If you update the flavourColors property on the donut series style, you can set your colors there.
With regard to annotations, probably the best approach to take will be to subclass SChartAnnotation, and then to add your pattern image as a sub-view of your annotation. They are basically just UIViews, so you have quite a lot of flexibility with what you can do with them.
Hope that helps!