Note that
dz = - d[t*f '(k)*k]
but now we take into account that k and f '(k) will change with t.
Furman was the first to point out to me that, for Cobb-Douglas, the correct answer is:
dw/dz = (1-α) / (1- α - t).
For a capital share and tax rate of 1/3, we get dw/dz = 2. Each dollar of a capital tax cut (dynamicly scored) raises wages by two dollars. See the derivation at Cochrane's blog. (Note that John and I have a slightly different notation, so don't be misled by the minus sign.)
What is the general case?
I have not worked it out, but I will offer a conjecture: You can write dw/dz as a function of the tax rate, the capital share, and the elasticity of substitution between capital and labor.
You can find a helpful hint in footnote 19 of this paper.
Update: Eðvarð I. Erlingsson offers up a solution.