Got it! Congrats on finding a workaround that gets what you want.
There are two feature requests that would be of interest.
One (that comes up with some frequency) is for Encore to take any video you choose for an animated button that is linked to a timeline. There could be a toggle of "take thumbnail video from linked chapter" (the current Encore method) or from some other timeline or asset in the project (using th eposter frame/duration method) or you specify the asset.
Another is to provide a drop zone, non-button, and add any video you want (or specify and timeline/asset, poster frame, duration etc).
Encore does not have an encouraging history for such feature improvements however. Most of us would settle for more error free processing/bug fixes.
