Getting Flash to be MSAA accessible is a chore in itself. And if you are converting a poorly written app to be accessible, welcome to hell :/
For my own reference, and due to the lack of any practical documentation on the net, here are a few things to keep in mind. Also, these were primarily tested with JAWS, a popular MSAA compliant screen reader. I'm just extemporising here, so feel free to correct any errors or recommend stuff I've left out (by adding a comment) and I'll edit it in..