Nope - nothing whatsoever to do with CSS!!
But I have found the answer - the define statement for the new button in includes/languages/english/YOUR_TEMPLATE/button_names.php can NOT be empty; you simply must have something - even if a non-existent image name or just a space, as the definition.
So this:
Code:
define('BUTTON_IMAGE_MORE_INFO', '');
causes the hover to malfunction...
and this
Code:
define('BUTTON_IMAGE_MORE_INFO', ' ');
or this
Code:
define('BUTTON_IMAGE_MORE_INFO', 'non-existent_image.gif');
allows it to function correctly.