[GeoffR]

The gap between the last line of text and the page number is made up of: a fixed built-in margin, plus any margin set by the publisher in the book, plus any blank lines due to the orphans/widows settings in the book. (Edit: The choice of font, font size, and line spacing can also affect the position of the last line of text, which can result in almost one extra blank line in the worst case.)

If you are considering converting to kepub to change the top/bottom margins, keep in mind that the built-in kepub bottom margin is exactly the same size as the built-in epub bottom margin. The kepub bottom margin only looks smaller because in comparison the kepub top margin is so much bigger, and because kepubs ignore orphan/widows settings.

So you should be able to get the same apparent bottom margin size for epubs as for kepubs just by changing the @page margins and orphans/widows settings in the book, which can be done automatically with Calibre.

However after all that, if you still want to actually reduce the size of the built-in bottom margin and still have the page number displayed the only option currently (for epubs or kepubs) is to patch the firmware.