Sorry to bump this, I couldn't find a list by searching, so I thought I would contribute. Two of the cameo appearances that I noticed recently and never saw listed before were Akari in the Dojo stage and Shingo in Isaka (EX Kyo) stage. Shingo here could be just a cosplayer, because the guy next to him is a Kyo-look-alike and they both look younger than the actual characters.
Japan:
Nakoruru - Middle left, standing next to the lion statue in the far back
Hinako - middle right
Shingo - Left side
Todoh - Left side, On top of the gate
Egypt:
Marco (Metal Slug) - left side, among the crowd
Fiona - right side, on top of the snake statue
Mars people - middle left, under the snake statue in the far back
India:
Cheng Senzan - middle, among the crowd. Holding a sparkling thing?
Chang: middle right, among the crowds
London:
Yamazaki poster - middle right, posted on a bus
B. Jenet - middle left, on a bus
Chris - right side, peeking from behind a bus
China:
Xiangfei - middle left, rooftop of a building
Tung - right side, sleeping behind the guy with the big masks
Lin - right side, on top of one of the buildings in the back
Brazil:
Butt - right side
Chamcham - middle left
Tizoc - middle right, behind the crowd
Ruins:
Charlot - left side
Kain, Shinnosuke - middle right
France:
Seth, Venessa, Ramon - right side
Shermie - left side
USA:
Duck's chick - wandering on the left half of the screen
Galford - right side, hanging on a metal structure
Mr. Big ad on a flying ballon
Duck King - in one of the choppers in the back
Stadium:
(none spotted)
Stadium rooftop stage:
Hanzo (Samurai Showdown) - left side
Billy's Stage:
Lilly Kane - left side
Krauser, Lawrence, Axel - mid screen
Dojo:
Butt
Hayate - mid screen
Kasumi - peeking from the window on the right
Eiji - peeking from the upper left corner of the window on the right (above where Kasumi is, hard to see)
Malin - midscreen, randomly peeking from a drawer behind the decoration swords.
Alley:
Mr. Big, Karman Cole - middle
Jack (AoF) - on a poster, middle right
Desert stage:
Wang Koh-san - left side
PaoPao Cafe:
Richard: - middle left, playing on a drum
Bao - Mid right, on top of the palm tree. Hard to spot.
Sky Noah stage:
Adelheid, Rose
Boss Saiki stage:
(none spotted)
Dark Ash stage:
(none spotted)
EX Iori stage:
Heavy D, Lucky, Brian: - Left side, besides the truck
Candy Diamond - right side, laying on a red car behind the barrel. You can see her eyes glowing.
Rocky (Robo Army and alternative striker in KOF2000) - right side, laying on s red car just to the left of where the barrel is. You can see his eyes glowing.
Isaka Stage
Shingo - right side, cheering from the building in the back, holding a sign.
Haohmaru - left side, at night only.
Mr Karate stage:
Shiki - mid screen, leaning on a pillar
Moriya - left side, standing behind the altar