He makes an appearance in the Homeric Hymns and Pindar, but doesn’t show up in the Iliad/Odyssey, that I’m aware of (but correct me if I’m wrong). Was the Arcadian influence on Homer that limited?
Yes, I think you’re right: Pan doesn’t appear in the Iliad or the Odyssey. I can’t tell you why; all my books are in cardboard boxes, but the Classical Dictionary might have something on this and on Pan in general, and I’d also check the Homer Encyclopedia and even Lexikon des frühgriechischen Epos.
I checked the Classical dictionary, and it had some general information on Pan, though no mention of his absence from the Iliad/Odyssey. It mentions that Pindar’s “piety embraced quite minor deities,” to explain the references there. The implication of the article is simply that Pan was a very localized Arcadian cult figure, with no relation to the Olympian gods. Athens adopted him and gave him a shrine in the year of Marathon, and there are no references in the rest of Greece until the 4th century.
Pan would strike me as a good candidate for a pre-Olympian/pre-“Greek” holdover.