I was playing with making Quartz Composer effects for iMovie last night when I made an interesting (though somewhat obvious) discovery. The “Audio FX” section of iMovie HD 6 is actually a number of AudioUnit components hidden inside the iMovie application bundle.
Go to your Applications folder, and do a “Show Package Contents” on iMovie HD. From there, you can find the AUs in the Contents/Resources directory. Copy all the .components to /Library/Audio/Plug-Ins/Components/. Done.