As a startup it doesn't quite work, as the large companies that will want that info.
As a feature added to an existing gateway - say Amazon or Google - it could allow anonymized demographics whilst not exposing you to all kinds of data protection issues.
One of the main reasons for using a payment gateway is to avoid the legal requirements for card handling, it seems like a logical extension to me.
I also hadn't realised Stallman wrote for the Guardian, but going through his historic articles shows a healthy number of articles.