I actually keep records of the update types anyway, so I had them on hand.
I wouldn't count on a boss, though... It's not really an official update type yet, just something I added since it seemed like it might have potential.
Also, here are the most recent dates of updates going by series:
F-Zero - 7-2, Samurai Goroh (assist)
Metroid - 7-19, Zero Suit Samus
Mother - 8-10, Franklin Badge
Legend of Zelda - 8-20, Link's moves
Animal Crossing - 8-23, Go K.K. Rider! (music)
Yoshi - 8-23, Yoshi's final smash
Star Fox - 8-28, Andross (assist)
Wario - 8-31, Wario Ware (stage)
Donkey Kong - 9-3, Diddy Kong's moves
Kid Icarus - 9-3, Pit's Descent movie
Fire Emblem - 9-11, Lyn (assist)
Kirby - 9-12, Battleship Halberd
Ice Climbers - 9-14, Ice Climbers
Mario - 9-17, Golden Hammer
Pokemon - 9-20, Piplup (Poke Ball)
Metal Gear - 9-21, Snake profile/MGS4 Love Theme/Snake's moves
We're way overdue on F-Zero and Metroid content. But it's worth mentioning that Metroid has had four updates (Ridley music, Samus, her final smash, and ZSS) while F-Zero and Mother have only had one each.