Bottom line – just need to rethink a bit. :\
So – I use Flickr Downloadr to handle the downloads, but it copies the title and description into a separate json file. I got to spend several hours beating my head against a brick wall until I finally found the correct syntax and a bash for-loop to extract this metadata from the json files and insert it into the image files. Now when I upload them, Flickr sees the metadata in the correct fields and auto-populates the Title and Description and all is well in the world.
Code has been added to my Code Snippets page, if you ever find yourself in a similar place…