Filson once sold dry tin cloth dusters. Now they would be a special order item but it might fit the bill.

I have one and while I much prefer the waxed shelter cloth version you could give their custom order folks a call.

Here is the waxed shelter cloth...



I carry a poncho for similar reasons and think you may want to consider that. My dry tin cloth duster just is not weatherproof enough for me in this application. A good silnylon poncho or even a traditional coated nylon one provides good walking shelter and good staying shelter. A poncho does need to be coupled with a pair of chaps or rain pants for total protection though.