Dumb question, but: is there a way to find/filter ones that are? (I can't seem to find anything in the (web) UI that makes it clear which books are downloadable.)

There wasn't when I went through my collection. Reading the announcement from Amazon it looks like the existing DRM free books will not be automatically flagged to be downloadable.

The publisher/author will have to go through a process to have their books be downloadable again.