I'm very addicted to alt-f2 and I'd like to use it for as much as I can. Is it possible to use it for the actions I mention in the subject? I see there is a plugin for power management with PowerDevil, but I don't know how it's used or if it will do what I want.

What is your task? I don't understand this. Do you want an extra menu to handle every task? How do you plan to decide, which action is required?
The plasmoid you mentioned can be configured to get focus by pressing alt+f2 in its settings.

cu

I'm afraid I was unclear; what I want is to use KRunner to put my system to sleep etc. Now I found http://userbase.kde.org/Tutorials/Krunner but it only details changing screen brightness and various options. Perhaps what I want is not possible yet.

If the features you wish are not possible, you may file a feature request at bugs.kde.org after ensuring one does not already exist. Alternately you could check at kde-look.org and kde-apps.org to see if a runner has already been written.

krunner currently supports "most" of the actions you request with the <powerdevil> krunner. The bad part is that it's a little "picky" and by this I mean:

• suspend : type "suspend to (disk or ram)"
• shutdown: type "shutdown" (only whole word works for me)
• logout : type: logout (again whole word)

maybe you could open a bug report/request to make this actions a little more general meaning to be able to recognize commands such as "turn off" or "sleep" as shutdown / suspend to ram.

hasta la vista