Herschel studied this area and feature so much I don't think you can write it off on the basis of one tract. He wrote volumes on it over many years. The 1837 observation of the sudden brightening of Eta Argus (Carina) might hold something - a tantalising piece in a bio on the Herschel family has "Key-hole" in quotation marks in line with various other small quotes from his 1837 observation. Maybe he did coin it after all?
The hunt should go on IMO!
Cheers -