I saw on http://goodies.xfce.org/projects/applications/xfce4-power-manager that xfce4-power-manager provides an Inhibit DBus interface which allows applications to prevent automatic sleep actions via the power manager. I grabbed the source code for the latest release (version 1.0.10) and discovered that the DBus interfaces are disabled by default. The interfaces can be re-enabled with supplying the --enable-maintainer-mode to ./configure Ok, so I build the code and I get the following: ... Making all in src make[3]: Entering directory `/home/xxx/xfce4-power-manager/xfce4-power-manager-1.0.10/src' make[3]: *** No rule to make target `org.xfce.Power.Manager.xml', needed by `xfce-power-manager-dbus-server.h'. Stop. make[3]: Leaving directory `/home/xxx/xfce4-power-manager/xfce4-power-manager-1.0.10/src' make[2]: *** [all-recursive] Error 1 ... A search in the code shows that the file org.xfce.Power.Manager.xml is in fact missing from the release.
This compilation bug has been fixed meanwhile, this file is part of git master (and maybe also earlier releases).