Thanks Rick, that’s wonderful info!
The Set Channels/Matte approach is cool, and I will make use of those in the future I’m sure. What you suggested worked great for me (though, curiously, if I set the down-sample to “quarter” in the composition viewer, setting the alpha to “full” stops working… is that a bug?), and it’s a good exercise in understanding layer render order.
But if the duplication workflow is efficient and a common approach, I’ll stick to that — it is straightforward.
Thanks again, I appreciate the help!