Toad -
I don't think step columns are ever officially mentioned. But, this is a good example where the DA method (with K=1.0) will probably be better / more reliable for ensuring stability than a K factor.
I'd use K = 1.0, but set the unbraced length as the distance between bracing... even if that is greater than the segment length.
There is another method to use for calculating the buckling capacity of this type of member. The method of successive approximations. This is described in the AISC design guide (number 25?) for tapered members. The method comes from one of Timoshenko's books (Elastic Stability?). And, the example that Timoshenko gives in his book is a stepped column.