I've tried that-- and I have been every bit as successful as you. I had hopes for the apps PowerPhotos and Photos Workbench, since they both show keywords, but you can't copy from them, either.
My guess is that the list of keywords doesn't actually exist. Rather, I think it may be generated on the fly from the database, and it only takes form on the screen. You can copy screen images and use Preview to copy the text from the images to TextEdit, but that means a screen at a time, and it can't be worth it.
I've been going through and changing all the "John Smiths" to "JohnSmith", and other fixes, but it's tedious--I've used so many keywords in the past, then changed my mind on just what I want, that it seems a hopeless mess.
Let us know if you think of anything!