I think that both would find some level of common ground in the idea that there is some inherent transcendental quality that exists in art. This means that art has some higher notion of the good that is to be revealed in its creation and pursuit. It is probably here where some divergence is present. For Plato, the artist must devote themselves to revealing the "form," or the ideal good that is present. It might be why he has such disdain for the poets who simply compose art for the public appeal of it and not for its intrinsic value of the revelation of the form. Thoreau is more along the lines of individual expression and if there is a higher "essence," it would have to lie in the very idea of individuals being able to listen to one's sense of self and state of being in the world. In this idea, art has to be a reflection of one's individuality, one's uniqueness that drives them apart from the conformist view of others. It is here that Thoreau is most zealous, and while the pursuit of the Platonic form is all well and good, Thoreau is more concerned with the individual's voice being acknowledged through art.