Swan* describes this kind of question tag as 'same-way' question tags, with this example: You're getting married, are you?.Non-negative question tags are quite common after affirmative sentences. These are often used as responses to something that has been said....The speaker repeats what he/she has just heard or learnt, and uses the tag to express interest, surprise, concern or some other reaction:
So you're getting married, are you? How nice.
So she thinks sh's going to become a doctor, does she? Well, well.
You think you're funny, do you?
'Same-way' tags can be used to ask questions. In this structure, we use the main sentence to make a guess, and then ask (in the tag) if it was correct.
Your mother's at home, is she?
You can eat shellfish, can you?
This is the last bus, is it?
[i]I'll...shall I?[/] can be used to make offers
[i]I'll hold that for you, shall I?
My feeling is that these same-way question tags are used more in British English than in American English.
In addition, certainly in American English, they often express hostility or present a challenge:
So you think you're going to marry my daughter, do you?
You want me to do your work for you, do you?
Jack said that about me, did he? We'll, he'd better apologize!
*Practical English Usage, by Michael Swan. Oxford 2005