According to the openbox documentation,
When this is enabled, and xinerama is running, new windows will not be placed in dead zones (areas of the desktop which are not visible because of different screen resolutions), or across screen boundaries. Note: in order to you use, you must also use the --enable-xinerama configuration flag when compiling Openbox.
example: session.xineramaSupport.windowPlacement: True
So, is this available on the pdaxrom build?