At the moment the xfce4-power-manager seems to use upower to act.Most modern system use systemd today and systemd offers it's own suspend/hibernate solution which is not only quicker than the upower alternative but less error prone (at least when you are actually using systemd ;))
So I propose that a feature can be added where you either specify a command that should be executed when for instance close the lid of your laptop or a function where you can specify your own commands for suspend and co)