Thank you guys so much for the outpouring of information and advice.

Ultimately, I decided to avoid any risk of an issue and took a different approach. Basically, we're going to include a dedication along these lines:

"This book owes much to the traditional ballads of Ireland and Appalachia. To the musicians who keep alive those old songs of horse thieves and highwaymen, lovers and lonesome pines—thank you."

I have zero musical talent or skill, and in some way the novel is my own version/performance of the music. I've done the same in the past, with short stories loosely based on "I Know You Rider," "In the Pines," and of course "When First Unto This Country." I think such a dedication communicates this appreciation and influence perhaps even better than the proposed epigraph, you know?