Victory is mine! (at least in mozilla).
getComputedStyle (on moz) and offsetHeight (proprietary MS bs) saved the day, allowing me to grab calculated height values for the main split-pane widget and use those to set a height value for the bottom pane in the horizontal configuration.
I have no idea how (or even if) the linked sample page is working on IE, so if you find that it's borked please let me know, providing browser version, your email address, and a general description (or screenshot) of the problem.
Now about that data table widget...