My T doesn't do dxs too- he says they are too limiting and he likes to treat clients as they are not by they dx.
As much as I wonder why this is so important for you I think you should feel free to discuss it with your T. It's true that BPD patients are considered to be the most challenging by Ts. IDK why... but I don't think it's not right to label clients like this and it'd tell me more about the T if he refused to treat a BDP client just because of his/hers dx than about the client himself. And I would not like to be treated by this T.