Yeah it hasn't happened often enough for a situation to have come up where MLB denies the application. Only way I could see it happening is if they tried to fake an injury.
But you still have a 40-man roster in addition to a playoff roster. So if you did theoretically want to call up Irvin to replace Eickhoff, you would also have to remove someone from the 40 to add Irvin (since Eickhoff, in this situation, would still not be taking up a spot on it).