1) load a blankMC from the library in to a custom component.
The custom component is made up of a window component.
2) from the blankMC, create a maskMC and a containerMC
3) load an external swf in to the containerMC
4) use maskMC as containerMC's mask
I have completed all of this inside Flash authoring
If i run my swf as a stand alone, the masking no longer works
for certain swfs.