"how does a DWR windshirt (e.g. Houdini, Sirocco, etc.) hold up to that sort of low volume/long duration drizzle? I'd expect that for me, full rain gear wouldn't be sufficiently breathable in such conditions."
It won't, pure and simple. The reality is that, either with or without rain gear, you're going to get wet in a classic all day PNW rain/drizzle. The difference is that with full rain gear you will be wet and warm, but without it you will be cold, wet, and at serious risk of hypothermia. It happens all the time up here. A personal example from last weekend: I was out on a 12 miler that started out in a slight drizzle. I hiked without my rain gear for the first 2 hours, but as we gained elevation the drizzle turned into soaking, increasingly cold rain and I started to shiver. I donned my rain shell and very quickly warmed up. I was just as wet as I would have been without the shell, but comfortably warm. Venting kept the heat from building up to an uncomfortable level. I would not have wanted to try and finish that hike without my shell. This is a very typical situation up here.
Edited: Footnote-a windshirt would not have lasted 10 minutes in that rain, and I would have been in a very tenuous situation, with at least an hour and a half hike back to the car when I was getting cold. Hypothermia would have been a serious concern.