In the Index novels you have the HO movie that's referenced twice (Touma talking about a recent remake is likely referencing the anime series), the Zashiki Warashi show Misaki watches in NT11 and I think there has been novel covers for HO and Blood Sign in the background of some color illustrations.
All the HO mangas have cameos of other Kamachi characters as part of the 37th Battalion. It started with Index characters (particularly Uiharu, Tsuchimikado and Misaka Imouto making repeated appearances) but
Shinobu and Yukari from Zashiki Warashi and
Kyousuke + White Queen from Blood Sign have also made cameos in the newest manga.
Zashiki Warashi has a HO LN cover as the cover of a PSP Dragon Hunter-like game about humans fighting giant rampaging machines. Mai Hishigami's outfit in vol 3 might be a reference to Index's Oriana Thomson's first outfit in the Daihaseisai arc.
Boo Boo has the reference to Zashiki Warashi with a police drama with Hayabusa and Enbi. Preview of vol 5 might have a reference to his Vampire/Zombie webnovel, but then again Maxwell is a cliche name for AIs/supercomputers. It also has a succubus with the exact same name as a demon in Zashiki Warashi, which Kamachi claims it's a coincidence.
Vampire/Zombie webnovel has a bunny girl announcer working for the evil bad guys named Karen, just like in A Simple Series. It also references Boo Boo via Ayumi's camisole, though you can only see this in the character designs due to lack of illustrations and no description of the design in the text, so that might be just Mahaya.