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.