Well, to sort of answer my own question, affter another 40 minutes on the phone and escalating as far as I could escalate (through both the Star Alliance desk and a customer service supervisor), I was informed that even if Lacsa is now in Star, United's system has no way to book a LR flight. They said they would report internally, but no one could give me a call back if and when the glitch is fixed. Great customer service from the new UA!