Yes, May returns during the Sinnoh season for the Wallace Cup competition where she meets up with Ash and Brock again.
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Dawn meets May in Episode 544 which is titled "A Full Course Tag Battle" which is part of the Diamond and Pearl Battle Dimension season.
The only trainers that you will meet that hold items are trainers in Battle Frontier they only start holding items once you get later in the obstacle.
Dawn doesn't meet May in an episode during the Galactic Battles season, she meets her in a Pokémon episode titled "Our Cup Runneth Over" which is during the "Battle Dimension" season.
Ash meets Brock in episode 5 of season 1, Showdown in Pewter City.