FeedBooks does something like this for ePub, using padding-top: in its title.css. Download any FeedBooks ePub and unzip it to see how it is done. However, this may not be possible in Book Designer. Alternatively, Adobe in its
EPUB Best Practices Guide suggests using SVG graphics to build the cover page, including the title and author text. This definitely isn't possible in Book Designer.
If you do build an ePub cover page, Calibre's epub-meta command has a --get-cover option which extracts the 1st page of the ePub as a 600x800 JPG image. This would be one way to get a cover image for non-ePub versions.