I've just noticed this 'long' thread. It is of interest to me as I had a not dissimilar problem. The use of superposition is absolutely correct in a linear system where the system component values do not vary with the signal applies. However, when applying the ac component the skin effect phenomenon will deplete the current carrying carriers from the centre of the conductive component thus reducing the effective cross sectional area......which will also effect the dc resistance....or will it?
Danny