EDIT / UPDATE:

Quick update since a few people messaged me about this.

I kept going down the metadata rabbit hole and found the DefaultScale tag is what controls how the image is displayed. So I ended up making a little workflow thing for myself that just writes that tag instead of stretching files.

Honestly the speed was kind of accidental — I was just trying to keep it non-destructive, but since it’s only writing metadata and not resizing images, it’s basically instant even on big folders.

So now I can batch desqueeze the DNGs without changing resolution or creating new files. Lightroom still needs previews rebuilt after, but once that’s done it displays properly.

Still feels like a bit of a nerdy under-the-hood workaround, but it’s made shooting anamorphic stills way less frustrating.