The code you provide works perfectly in ADE (after fixing the double brace error:
Code:
p.figure_caption {text-align: center; font-weight: bold; font-size: .9em; page-break-before: avoid;}
)
The iPad is 1024x768, and iBooks adds enough UI fluff and margin padding that I doubt there'll be any display area left over from a 680x800 image.