If the object is automatically placed in a p (or div), centring the object via the centre/center! align button on the WYSIWYG editor should do the trick. Objects themselves don't centre, so I understand the use of p. Some blogging platforms allow you to chose between p and div for new input. Both should work with the align buttons.
@ardav and @nizuya: guys, thanks a ton for the input. I actually figured out how to make it work and it was super-simple. I just needed to set the OBJECT to display:block and then the margin:auto worked. Don't know why I didn't think of that sooner.