Well, both created fictional painters active in 1926. Lovecraft came up with Monsieur Ardois-Bonnot, a French painter who “… hangs a blasphemous Dream Landscape in the Paris spring salon of 1926”, for his short story The Call of Cthulhu.
Ardois-Bonnot is an example of the worldwide psychic disturbances signifying the reappearance of the alien god. Cthulhu is obviously an art lover.
Waugh’s contribution is Monsieur Jean de Brissac la Motte who appears in his novel Brideshead Revisited. De Brissac la Motte joins the narrator and friends in London at the time of the 1926 General Strike. He claims to have been in Budapest after WWI when Admiral Horthy rolled in and crushed the Bolsheviks; but ends up in hospital after an elderly widow drops a flower pot on his head from a top floor window.
“We were joined by a Belgian Futurist, who lived under the, I think, assumed name of Jean de Brissac la Motte, and claimed the right to bear arms in any battle anywhere against the lower classes.”