I have what's becoming a serious problem in my business and wondered if you'd encountered it, since you lived and worked up here for so long. And how did you handle it?
Many of the brokers in this area are weather chasers. They used to call to book a shoot and say things like; "dependent on the weather of course". To work around that I decided that every photo on my front page slider would have a fake sky in it... Both inside and out. Now, when they worry about the weather, I tell them the photos they've been admiring were all taken in gray skies (most while it was actually raining, sometimes hard). It hasn't worked.
I think when you shoot in Seattle or any place other than Arizona or the Sahara desert, real estate photographers need to be prepared to do sky replacement on rainy days and have a clearly stated policy about not canceling shoots because of rain. Dan Achatz's YouTube video on this subject clearly demonstrates that it's possible to produce great real estate exterior shots in Seattle in bad weather. I don't know if Dan charges extra for sky replacement, I doubt he does. In Seattle, rain all the time is just a fact of life. You'll have to decide if it makes sense to charge for sky replacement in your situation. Not having to reschedule shoots all the time will probably save you enough time to just do sky replacement for free. It doesn't take that much time once you get the knack of it.
Oh, and here is the PFRE sky library if you need some sunny skies.