Lewd, Lascivious Lips

Brian Thomson has always maintained the disembodied lips were inspired by a Man Ray painting. Though disembodied lips were hardly an original idea by 1974 - and he could have been influenced by many other works - on canvas, stage, or film. Or maybe those other works were also inspired by Man Ray?

Observatory Time: The Lovers (1936)

Brian Thomson wanted to be a pop artist. He inserted dozens of art references in the set, and he would have been very aware of Andy Warhol's works. 

Thomson was working at The Royal Court theatre around the same time as the 15-minute production of "Not I" by Samuel Beckett  (about 5 months before rehearsals started for Rocky Horror). In the staged production Billie Whitelaw's lips were illuminated by a small, focused spotlight. Later that year there was a filmed version (broadcast in black & white) which had her face blacked out so only her lips were visible. Thomson would have certainly been aware of this production.

Not I (stage production)

Not I (televised production)

But even before that, Mouthscreen was a common cinematic trope. Some other films (before Rocky Horror) that have used the "Mouthscreen" technique include: 

Les Femmes aka The Vixen (1969)
The moving lips appear in the entirety of the 
opening and closing credits.

Bonnie & Clyde (1967)
Opening scene

Black Narcissus (1947)

Citizen Kane (1941)

In fact, Brian Thomson and director Jim Sharman even used a Mouthscreen shot in Shirley Thompson Versus the Aliens - for 86 seconds.

Shirley Thompson Versus the Aliens (1972)

The Rolling Stone's logo was designed by John Pasche in 1970. It has been called the most famous logo in the history of popular music. The logo has remained on all post-1970 albums.

Salvador Dali designed a Lips sofa with fashion designer Elsa Schiaparelli - 5 versions of the couch were produced in different textiles.

And then.... you had these hilarious novelties. Wax Lips. Invented "some time in the mid-20th century", their popularity piqued in the 70s.