You can include pages that don't have a header or footer, by creating a Topic containing the cover page content and then adding a "Booklet Cover Page" entry to the Table of Contents or the "Content" page in the Booklet editor.
Add a Booklet Cover Page node to the Table of Contents
Topics referenced by a "Booklet Cover Page" node will not have a header or footer in the generated output. Cover page topics will have a title, but you can set the title of your topic to a single space if you don't want a title included. You can include any content you like in a Cover Page Topic, including for example a large image and can use any formatting you would normally use in HTML - e.g. centered text, large font sizes etc.
Add the cover page entry first in the Table of Contents to have it output as the first page in the Booklet.
In our latest versions (2016 onwards), you can optionally include either the header or the footer to the cover page by right-clicking the table of contents node and selecting "Edit". From there, you can select these features individually - this is useful in case you only wished to show the footer - or only show the heading, and you can also optionally set the "Full Page (No Margins) setting.