One of my upcoming flights has a mish-mash of Star Alliance airlines. I booked thru United, but there's also Air Canada and Lufhansa segments.
When booking seats on united.com, I was able to book all except the Lufthansa segment. So, I went to Lufthansa's web site and wasn't able to book it there. I tried calling and got the standard, "our phone lines are unusually busy at this time..." and I hung up. I called United (I have status with them, so got right through), but the agent told me aren't able to book Lufthansa seats. On a lark, I went to Air Canada's web site, brought up my flights, and was able to book the Lufthansa seat there! I doubt this would have worked if I didn't have an Air Cinda segment, but it's worth a try. Sure beats staying on hold forever.