The inserted characters have all been rendered in Natsu no Arashi’s rather minimalistic style, but they should all remain readily recognizable:
Idol Master girls.
Hinako, from the recent Training With Hinako Fitness / Ero-Anime
Haru-chan, seated, a mascot character of sorts used in NHK’s televised weather forecasts (her name translates as Spring).
Maria Holic, which also had a bevy of cameos in its finale.
Saki’s girls, albeit camoflauged with subdued hair colors.
Most of the cameos were quite brief, and likely invisible to any commoners that might have tuned in (save for Haru-chan), so the creators of the show seem most aware of their show’s target audience.