Paul,
There was an article about a loaded vertical for 160m in AR a few years ago, ie. in the last decade, I think by Peter 3YE. He found that a loading coil towards the top of a vertical, with a substantial top hat of several wires going out radially and connected above the loading coil, seemed to work well. I think that may have been the basic design of the vertical made by Tony VK3CAT and used successfully on a SOTA activation with a reasonable ground radial set.
The impedance of the double sized zs6bkw on 160 would be predictably reactive and a high total impedance, based on the impedance of the standard size on 80m, giving an swr of about 8 at the bottom end of the feedline, on mine anyway. If 160 is the only purpose of the antenna, you could forget the feedline length selected by the original designer and go with whatever you found worked. The impedance of the doublet on 1.8 mhz would be quite low, perhaps only several ohms but the reactance quite high. Tuning could be expected to be critical. I suspect the main contribution of the fence to the antenna impedance is a whole lot of ohms. That will broadband any antenna!
Let us know how you go. There will be plenty of observers!
73 Andrew VK1DA/VK2UH