This has been discussed over at WPUniverse. A few have found add on progs to do this. The threads are pretty old, though. Put without the quotes, "site:wpuniverse.com side ruler" or "site:wpuniverse.com vertical ruler" as the google search term and you can find these discussions.
I would bet a macro guru could create a macro to find the height between the top and bottom margins. There are macro gurus that hang out at wpuniverse. Barry MacDonnell, who occasionally posts here, is the most active member over there.
Though a pain, you could get the top and bottom margin from the 'hot marks' when hovering over the margins, and do the math.
Sorry I have nothing further.