User blog comment:ExtremeVelocity/Cameos!/@comment-1784397-20130813181324/@comment-30764484-20130815060914

Not a cameo, but there are quite a few references to K-On! in the first episode of Tokurei Sochi Dantai Stella Jo-Gakuin Kōtō-ka C3-Bu (or just C3-Bu).

When the girls of the C3 club are trying to get main character Yura to join the club, they are all eating cake and drinking tea. At one point Yura thinks to herself "Oh I get it, this is one of those clubs that just sits around and drinks tea, and never does any club activities", though that doesn't turn out to be true.

The most obvious is when Yachiyo (voiced by Madoka Yonezawa, who also voices Ui) suggests, as a means of getting Yura to join, slapping her face with a wad of cash.