How about some Sunday morning sausages?
In 1979, Strayhorn had a big hit with "Heartaches Of A Fool". As a thank you for rescuing his public image, he cut a personalized single of the song with a thank you on the label to Joe "Rocky" Rockford.
Whenever the song was heard during the episode, we didn't see or hear Charlie singing it. Instead we heard Willie Nelson who actually wrote it.
Three years after the episode aired, Nelson reached #39 with the song in December, 1981. (It can be found on the album "Greatest Hits And Some That Will Be."
Willie Nelson achieved televersion immortality by playing himself as a murder suspect in an episode of 'Monk'. (He also has a Tooniverse counterpart thanks to 'King Of The Hill'.) So it could be that in Toobworld, Nelson only recorded a cover of Strayhorn's song.....
Here's Willie Nelson singing as Charlie Strayhorn in the opening sequence from 'The Rockford Files' - "Heartaches Of A Fool":