I figured it out, after what is now a couple days of digging through criptic DBus documentation and wiki.maemo.org (and a bunch of random plug-n-play trial-and-error on the command line):

dbus-send --system --print-reply --dest=com.nokia.mce /com/nokia/mce/request com.nokia.mce.request.get_device_orientation

In retrospect, the python snippet that I initially found on the wiki and you posted here contains most of that in it. But there's no way I would have gotten it without seeing a bunch of other dbus-send code snippets and reading through dbus documentation.