At first I thought this wouldn't work, but I've convinced myself it does. You might add a comment saying that setting poster.blob to the file blob alters the data strucutre stored by the filmstrip module, so previews of this video will used the file-based version rather than the memory-backed one.

And if you're feeling ambitious, you can edit lib/get-video-meta-data.js to remove the spurious comments that say the poster image is saved, when clearly it is not.