Namco's Taiko no Tatsujin appears in Sega's upcoming gangster opus. When you visit an arcade in Yakuza 5, you can play the Namco rhythm game.
Sure, Japanese game companies often loan characters to other franchises—whether that's Monster Hunter appearing in Metal Gear Solid or Sonic and Mario doing video games together.
However, this is one developer's game appearing in another developer's game. I'm sure there are previous examples of this—or examples in other examples. Or something.
『龍が如く5』と『太鼓の達人』がコラボ ― 神室町でプレイ可能 [インサイド]