lesspopmorefizz: For anyone else experiencing this issue:
Support automated email pointed out the www.gog.com/account/refresh URL to me, which, shockingly, actually worked - to a point.
The DLC now appears under the game when viewing my library on the gog.com web page. Cool! GOG agrees with me that I own DLC for this game!
Unfortunately, the DLC doesn't appear in Galaxy, and, worse yet, the download link on the web page in my library goes to a 404. (the other DLC links and the game installer links work fine through the web page. It's just the Inevitable Excess link that goes to a 404.)
Do you still have this issue? GOG replied to us that the issue should be fixed.