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>