Early this month I made a UA Mileage+ rewards booking from SFO to DPS and CMB on Eva and Thai. The trip is in Dec 2014 and the return in Jan 2015.I was able to get the Eva conformation number from UA and then select my seats on the Eva website for my SFO-TPE, TPE-DPS, and TPE-SFO flights. So it was almost 300 days out.
I believe both Eva and Thai use Amadeus for their reservation system, so the confirmation number for Thai was the same as Eva. When I logged on to the Thai website, I could see the Eva seats I had selected, but I was not able to select seats for the Thai flights. But I just sent email to
[email protected] requesting isle seats for all the Thai legs, and they did it within a few hours.