Perhaps I skimmed too quickly through this thread but in MY case, I simply added another Dolphin icon to my Panel, right clicked on the icon, selected "icon settings" and when that opened, I went to the "application" tab, then opened the "advanced options" button, then under the "user" section, selected "run as a different user" and put "root" as the other user. I then went back and selected another icon for this so that I don't get it confused with the regular Dolphin icon. When I click on the new Dolphin (SU) icon I created, the root password box pops up and as soon as I enter the password, Dolphin opens with full root privileges.
Hope this helps someone out....
Regards
Christian