Remember that even when MIN or MAX modes are active, the src and dst multipliers are still active (like FIXEDA or GL_SRC_ALPHA or whatever) , so you need to implement all those too. 

The normal blend equation (GL_ADD) that we don't normally see because it's implemented in hardware is:

     src_pixel * SRC_FACTOR + dst_pixel * DST_FACTOR

The min mode changes this to 

     MIN(src_pixel * SRC_FACTOR, dst_pixel * DST_FACTOR)

and so on.