A poster reading "LeGuinness" also appears at Orion's Belt, although it may be difficult to spot from the darkness.
The drink is a play on Guinness stout (a type of beer) and the fantasy author Ursula K. LeGuin, author of the Earthsea series. In 1994 (the year before SQ6 was released) she won the Nebula Award for the short story "Solitude", one of four such awards she has won.
Just like with Heinleinekin's, the name of the drink is spelled differently on the poster. Judging by the origin of the reference, "LeGuinness" (with two Ns) would probably be the correct version.