There was a soundtrack album for "Outlaw Blues" but it's apparently out of print. AMG lists Steven Fromholz as the artist, but it doesn't give a track listing. The Internet Movie Database (IMDb) doesn't give a complete soundtrack listing either, but it does list Hoyt Axton as one of the contributors of original music. TV Guide Online gives this somewhat ambiguous information:

Songs include "Outlaw Blues" (John Oates), "Jailbirds Can't Fly" (Harlan Sanders, R.C. O'Leary); "Whisper in a Velvet Night" (Lee Clayton); "I Dream of Highways," "Beyond These Walls," "Water for My Horses" (Hoyt Axton), all sung by Fonda.

This Hoyt Axton Discography gives a 31-second sound sample, and says that Hoyt sings the song on the soundtrack album. I transcribed this:

Whisper in a velvet night brings to me your name
Touch me like the sunlight through a crystal windowpane
Warm me like the memory of a dim and distant day
Move me like the moonlight on a mountain far away