I don’t know if this is documented anywhere, but I found that it’s possible to create a smart album to track photos that have been uploaded to Flickr using the FlickrExport plugin.
FlickrExport adds a metadata tag with the Flickr URL when it uploads a photo. When you create a smart album, one criteria you can use is ‘other metadata’, which you can select from the ‘+’ popup in the top right of the window. One of the metadata options will be ‘Flickr URL’. Choosing Flickr URL is not empty will select all photos which have been uploaded to Flickr. Alternately, ‘is empty’ will select photos which haven’t been uploaded. Combining these options with star ratings make it easy to choose photos to upload when you have a lot of them.
Using that trick I managed to finish going through the rest of my photowalk photos and uploaded many more today.