I just uploaded 
openbox-3.4.7-2 to 
Debian unstable and wrote a patch that enables to set 
next or 
prev to the 
monitor attribute of the 
MoveResizeTo action. Without this patch you need to explicitly set a monitor number you want to send a window to but can't just toggle a window from one workspace to the other with one keybinding or to move it to the next/prev if you have 3 or more monitors. So if you use openbox with a xinerama setup it would be nice if you could test that and report bugs in case of errors.
A keybinding for that could like like:
- <keybind key="A-m"> 
-   <action name="MoveResizeTo"> 
-     <monitor>next</monitor> 
-   </action> 
- </keybind>