I suppose you could say:

a) LARGE braking forces go through the spokes for both disk brakes and rim brakes.
b) braking force as in slowing down the mass of the bike, does not go through the boss, except for some of it in the case of chain-stay mounted brakes. In practice it's hard to assign responsibility to forces since they act in sum, but the force at the fork bosses does the opposite of this.