If May were to return to the series as a cameo/travelling partner in the new region, how would Ash react?
Probably no reaction at all, just happy to see her I guess. Unless she got there before Ash and maybe we would see his reaction.
Are there any notable characters in the Anime who you think would be supportive of Ash and May being a couple?
Not really. All the characters see them as good friends (excluding The Bicker, The Better).