I had an itinerary recently where the first flight was UA, so OLCI was through UA - second segment was LX and I was not able to select a seat during OLCI. It was assigned at the airport. The flight's probably empty anyway, I wouldn't worry too much about getting a bad seat assigned. I wasn't actually able to select any seat after T-48 through the LX website whether I wanted to pay or not, but ended up getting one of the SEN/HON seats assigned automatically.