Even if you can select seats, you can't do much about it, you would only limited to select within a few rows. That's the price you pay for codeshare !
As for example UA will buy a block rows of seats usually toward the back of Y, that's how I learn flying UA codeshare on NRT-HKG-NRT. And even when I got to NRT at the NH lounge, I was being lied to, was told all other seats are occupied, they can only move me within those rows. Instead the plane was half empty !