In the crossovers with Gintama, the Yorozuya and the Sket Dan made sure they were self aware about who were their voice actors and their respective roles in each series.
In the first episode, Switch mentions that his favorite Visored is Muguruma Kensei, who Tomokazu Sugita also voices, and later in episode 31, Switch suggests (with a voice tone similar to Gintoki) to use a rather familiar white and black suit.
Executive Meddling: Cherry's introduction in the manga series. Roman mentions that his creation was a request from the editor to allow some fanservice (something that Shinohara often avoided) to appear in the series. And, oh boy, the Sket Dan knows that something is going to go wrong when he appears.
First Appearance: With the difference of exclusive characters in both one-shots, and the appearance of the Sket Dan (with minor design differences), Otakura and Yabasawa made their debuts in the first Akamaru Jump one shot, too.
In the Gintama crossover manga, Gin and Switch realize they sound the same and proceed to call each other "Sugita" for the remainder of the issue. In the anime, however, both Gintoki and Switch just refer themselves as "onii-chan".
Momoka kind of fits as metacasted, since Marina Inoue started at a young age (20 years old compared to 17-year-old Momoka) at the seiyuu and singing industry, too.
Dante shares some similarities and musical roots with Gackt.
Name's the Same: Chiaki Takahashi, often referred as Captain who shares name with seiyuu Chiaki Takahashi... except it's not Chiaking herself (nor in voice nor shared background).
The Other Darrin: As often happens when there are Drama CDs or VOMICs before an anime series, the cast changed along the way. The only ones to keep their voices were the Sket Dan, while the other characters got recasted. Yuu Kobayashi, for example, was given the role of Daisy, when she was originally Reiko in the Drama CD.