With minimal reasearch. The word "creach" can be roughly translated to the Old English "Hyre", meaning a downfall, descent or ruin. Is there any chance that Creach, Creacher, or Creachery, could in reference to his association with the National Brotherhood of Professional Baseball Players, and their opposition to the reserve clause? Also maybe combined the Buffalo incident in late 1888, where he refused to report to Pittsburgh(whom he'd been traded to) because he wanted to play for Buffalo(a team that he had purchased)? Perhaps his actions were viewed as destructive to the game by some people? This could also possibly have led to the Mc being added in a derogatory manner.
The Buffalo incident is briefly discussed in this article..
http://research.sabr.org/journals/james-deacon-white