I too got the run-around from 5 agents in different departments at US Air about getting LP mileage credit. Adding "LP" did not work.
Finally I spoke to the very helpful Jenny Graham of the US Air Consumer Affairs Department (336) 661-0061. She said she was able to find a Dividend supervisor who knew how to arrange the mileage credit. It seems that the process for LP credit is not automatic and so requires "a few extra steps."
I would appreciate hearing from others about information (i.e., verbal or writtin US Air confirmation) regarding getting US Air to LP credit.