Focus floating system windows with keyboard
If you enable full keyboard access in System Preferences, you can press tab to select the buttons.




I don't know how to focus the windows though. They can't be selected with ⌃F4 or ^F6. I was going to suggest Witch, but it doesn't show them either.

This is a bit of a hack, but you could assign a shortcut to a script like this:

tell application "System Events"
    repeat with p in {"SecurityAgent", "CoreServicesUIAgent", "loginwindow"}
        if exists process p then
            tell process p
                if windows is not {} then
                    set frontmost to true
                    exit repeat
                end if
            end tell
        end if
    end repeat
end tell


Password dialogs are shown by SecurityAgent, Gatekeeper and quarantine dialogs by CoreServicesUIAgent, and the force quit window by loginwindow.