Very helpful thread!  Thanks everyone.
I had this same thing happen.  I bought a brush blade and put it on last Fall.  Now Spring, I forgot that the top piece was not part of the accessory, and got the string head stuck as described.  I was by myself, so I used vise grips to grab the cable and weighted the pliers down with a spare truck tire to hold in place.  Worked well!
                
 Shane
                
 May 2016