According to Buford Jones in "’The Hall of Fantasy’ and the Early Hawthorne-Thoreau Relationship," Hawthorne’s letters were an effort to try to convince Sargent to accept Thoreau as a contributor to replace Hawthorne in Sargent’s New Monthly Magazine (1429).
Buford Jones also notes that Hawthorne’s letter to Epes Sargent reveals that the narrator’s companion-guide in "The Hall of Fantasy" "is a strikingly full, thinly disguised fictional portrait as [Thoreau] appeared to Hawthorne in the late summer or early fall of 1842" and that the narrator and his guide "assume the precise moral stances of Hawthorne and Thoreau concerning several of the most prominent philosophical and social issues of a reform-minded age" (1429).
Jones, Buford. “’The Hall of Fantasy’ and the Early Hawthorne-Thoreau Relationship.” PMLA
83.5 (Oct., 1968) 1429-1438.