Wario appeared in G&W Gallery 2 and 4 (both numbers are even numbers)
Speaking of the game, a few items from Wario Land also appear:
In the other game, it seems Wario is holding the Golden Hammer from Wrecking Crew, defending his statue...
Not much of a reference per se, but it is notable that this Mario Party 5 Wallpaper borrows a Wario World artwork :
This might not sound spectacular on it's own, but what makes it interesting is that Nintendo, for how much they reuse renders...