Bug Killer offers an elegant solution (to hold the front sprocket while the countershaft nut is removed).
A less elegant solution might lay a segment of the old chain across the tops of both front and rear sprockets for the same purpose.
A piece of wire or string might temporarily connect the ends of the cut chain, insuring it won't slip off the sprocket teeth.
I think also, an impact wrench is your friend in cases like this; you might not even need no stinkin' chain with reasonable engine compression and a good electric- or air-powered impact wrench, oh WAIT! The KACR! Impact wrench might beat the KACR to the punch, nevertheless.