I can imagine her singing Express Yourself, and then doing something shady whilst Born This Way fades the song out.
It will be boring if she actually just sings it.
I think it'll be friendly and respectful. Probably followed by a speech promoting unity and togetherness, etc. Hopefully with a Britney-like appearance later on.
Also, before people start. She does NOT, I repeat, NOT have to pay Gaga royalties or anything like that since it's only a cover on tour and isn't for sale.