I spent way more time than I really wanted researching Oregon boundary fence laws and turned up this state statute that explains that responsibility is 50/50. I wrote a letter explaining everything I was proposing and included a copy of the statute and the city's recommendation for building. I really didn't know how they would receive it so I tried to temper the formality by including a bit at the end about how I feel very warmly toward them and I really want to keep everything friendly.
And you know what? I got a super nice email that night in response. Huzzah! He even thinks he might prefer a horizontal board fence. DOUBLE HUZZAH.
So right now we're planning for May and then I can start focusing on camouflaging the fence on the west side where there's so little privacy it might as well be a window. I have a litmus test for how private I'd like the yard to be but I won't write it here since my mother is reading this.