Sometimes you don't need to ask the user directly because it is easy for them to say 'yes, I would use'.

Instead, you can place a button or menu that could drive to the feature and when clicked present a page saying 'the feature is under consideration, click for the waiting list and you will be first informed when it is available'.

As you will be monitoring the click and registering rate, this is a cheap, fast and reliable way to test if your users are really willing to use the new feature.

What you don't have, doesn't break. ;)