easyest way would be to have the character just as you normaly would. animate up to that point where arm gets cut off. thin swap the geo with a copy of that charicter riged but with geo missing for his arm.. and a copy of his arm with a small rig in it.. usefull for falling..

Makeing a character whos arm could fall off later. All in one character is rather dificult. Getting the seam and cut that could acure not to be visible or couse artifacts in the frames befor the arm gets cut of.

hope this all makes sence .