How do I use Flash SWF's to make an alpha (opacity) map?

You can use Flash-based textures and create very effective alpha maps just like you can with jpgs. The difference is in the approach. With jpg alpha's, you typically use two images- the texture jpg and the alpha jpg. If you're working in 3DStudio Max, you will apply one as the diffuse map and one as the opacity map. Of course, you can use the same image for both, but you will be better off using a grayscale version of the diffuse texture for your alpha texture.

With Flash textures, you only use the one SWF to do both. The part that is going to be transparent is simply an empty background. If you want to make that background black (or any other color) in order to see where the transparency will be, no problem. Note, however, that if you use a blue background, you will get blue lines at the edges of your alpha'd object. That's a current limitation of the plugin.

The great thing about using Flash textures with alpha's is that you can very easily change the alpha pattern by simply swapping in a new SWF with a different background pattern showing.