Interestingly I did manage to select seats on a recent UA booking, which has one LX segment (ZRH-LHR) - this segment is coded as an LX flight number, not a codeshare. However after selecting seats I'm now told I can't change seats anymore. (My selected seat propagated through to the UA side of the booking, I can see the seat on the UA website as well as the Swiss website).
I have another ticket, this time with AC, which includes a codeshared LX flight (i.e. AC flight number), I'm guessing I won't be able to select seats on that? (The ticket shows "airport control" for that segment, which is ominous.)
Finally, I need to book a standalone LON-ZRH/GVA flight, if I book a K class flight via the United website, does that mean I'll get free luggage? (They don't seem to have any of the "light" garbage.)