The Netherworld chapter actually has a very blatant reference to Metal Gear Solid. If you find yourself in jail, one of the few methods you can bust yourself out is to give the bottle of ketchup found in your cell to the guard, who'll give it a taste and then complains that it is "wretchfully rotten" before rushing straight toward the bathroom next door, dropping the key in the process.
Even funnier? That guard's name is also Johnny (well, actually Jonii, but close enough). Poor Johnny Sasaki, even in an entirely different game, he is still suffering from bowel problems...
