The black, demonic depiction of Mazinger Z in certain scenes is the titular demon of Mao Dante.
Episode 2: Most, if not all of the delinquents seen with Boss are random thugs from other Go Nagai works, such as Devilman.
Episode 7: In the flashback, we see photos of the archaeological team that was searching for Bardos Island. Many if not all of them are cameos from other Go Nagai works.
Episode 9: Tsubasa mentions a priest visiting from "Daemon in Abashiri." Daemon is one of the main characters from Go Nagai's manga Abashiri Family.
Episode 10: Ankokuji's line "Kurogane House is turning into a Shameless School!" is a reference to Go Nagai's breakout manga, Harenchi Gakuen (Shameless School).
Cross is partly based on Naojima Abashiri from Abashiri Family. Their cyborg forms are almost identical and both were voiced by Tessho Genda.
Episode 13 features various weapons from Devilman and Violence Jack, including Jack's knife, the Slum King's samurai armor and the anti-demon special forces armor.
Episode 13 also has Kouji call Count Brocken "Count Decap", which was his name in the Tranzor ZMacekre.
Episode 16 has Baron Ashura address the audience in the same manner as Akira from the later part of the Devilman manga.
Oddly enough theres a Ken Ishikawa reference in ep 19, Ashura appears very simillar to Ryoma Nagare from Getter Robo Armaggedon (who was in turn based on Shinichi from Maju Sensen)
Also in episode 19: the shot of Zeus holding his arm in his mouth is identical to what Jack does in the Golden City arc of Violence Jack. Since this was the arc that featured the original Tsubasa, it can't be a coincidence.
Episode 23 features a giant statue of Devilman in the hotsprings.
Episode 24: Similar to Boss' gang, the Kurogane Mob consists of random thugs who have appeared in various Go Nagai manga, such as Violence Jack.