Presuming you're using KDE4, you can try a command line call similar to the one's used here
Select allqdbus org.kde.powerdevil /modules/powerdevil setProfile <Profilename>
Replace with the name of a preconfigured profile you added (I think there's something like this called "Presentation" or something similar)
Came across today that dbus has an inhibit command i can use. Not the same thing but looks like powerdevil doesn't have the ability to disable power management.
qdbus org.freedesktop.PowerManagement /org/freedesktop/PowerManagement \