Here's a bizarre tip to try next time you want to preselect seats for a codeshare: in combination with the SK record locator UA gives you when your reservation is ticketed, enter your name on SKs website as FIRSTMIDDLE LAST rather than FIRST LAST. For some reason UA concatenates the passenger's first + middle names when they're sent to partner airlines. This was the only way I could access my UA-booked flights on LX or SK.

Of course YMMV!