Offering backing up whole blog

Hi, I am going to use Custom Content Dashboard Widget and have such idea, I'd like my users to see offer to download their blog's backup once a week.

To clarify - once a week, user will see in his/her dashboard (as a widget) some text which offers him/her to download backup of his/her blog and a link of downloading.

So, how can I do this? On Export page you have to choose what you'd like to download and click on download button and I want this to be automatically (so I want a link to generate and when user clicks on this link, he/she will start downloading a backup of site (with "All Content").

How to generate this link?

Thank you