I wrote to phase one directly and received this note from David Grover. I just played with this method this morning for the first time, but it does seem to work. I need a bit more time to play with it to see if I've got it right. This solution might be enough for my personal use. David also wrote that they are working to improve on key wording aspects.
Thanks for contacting us.
The way key wording is setup in Capture One right now is as you describe. I.e. The Filters tool shows you what is available in the current collection of images. I.e. An Album for example.
As a workaround you could select to view 'All Images' in your Catalog, then choose to filter by 'No Keywords' in the Keywords section of the filters tool. Then you will be able to see all your images that do not contain a keyword. To quickly keyword your images, you can just drag and drop them onto any keyword in the list.
Let me know if you need more assistance.