No, she did not go dressed as Buffy this time.

[fpb]

But there was something else that was surprising - if you are a JKR fan. She was wearing a decorous but figure-hugging brown jacket dress - and JKR once said that she hated yellow and brown (the colours of her old school uniform) and "would not wear them out of principle." (Telling Tales, an interview with J.K.Rowling, by Lindsey Fraser, Mammoth Publishing, London, 2000.)

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[fpb.livejournal.com]

Er, I never found any pictures of what she wore at the OotP presentation - so I cannot confirm my memory that she wore a Buffy-like outfit. As for the last outing, I could only find a very small photo (not wanting to spend hours googling everything in sight), where you can barely see what she is wearing. Here it is: