I'd suggest William Himes' "Concertino for Flugelhorn" - II. Ballad, Oskar Böhme's "Trumpet Concerto in F Minor" - II. Andante religioso, Tony Cliff's "Cool Contrasts: Two jazzy pieces for trumpet, cornet, or flugel horn and piano" I - Melancholy Mood, and William Perry's "Concerto for Trumpet and Orchestra" (flugelhorn) - II. Ballad.
Hope that this helps!