Found another Battle Frontier/Frontier Brain reference! You remember the scene which reveals Lisia and Wallace are related (for those curios, Lisia is the daughter of Wallace's older sister who had moved away from Sootopolis) and talking with her again will reveal some extra info. In one she mentions other trainers who are friends of her uncle. One is a man who likes stones (Steven) and another is "that trainer who looks like a star". A very vague reference and description, but with some thinking you'll realize this trainer is:
Tucker (its a bit more easier to figure out in Japan as his Japanese title is Dome Superstar)!
So so far the Battle Frontier reference count is as so:
1. Battle Tower model at Battle Resort which mentions the Battle Frontier project.
2. A guy in the Battle Maison mentions working for Scott and having found the Frontier Brain for the Battle Pike.
3. Nearby there's a female Delinquent who mentions having a Seviper, possibly hinting at her being Lucy.
4. Brandon had a PokeNav Plus show, having done a report on Volcanion.
5. After beating Lisia and Wallace in a Pokemon Contest, Lisia will mention a trainer who looks like a star, a possible reference to Tucker.
So that still leaves no mention of Noland, Greta, Spenser, and Anabel.