Right click on the file in Dolphin and select "Root Actions" and "Open as Text".
If you have already set kate as the default, it will use kate.
Other "Root Actions" are available ... E.g., can open "File Manager"(Dolphin) as root then select the file and "open with" ...
sudo <console text editor> </path/to/filename>
works too of course.
Sorry Strit, The method worked for me when I wrote it and the system was fully updated, even rebooted. I apologize for the noise.
Today, after not using ML for several days, I booted to it to, primarily, fix /etc/default/grub. To my surprise "Root Actions" no longer function. Dolphin cannot be run as root, kate, kwrite, ...etc. will not start.
Konqueror will run as root but it does little good for editing files. LibreOffice Writer will edit as root but that's such a huge overkill(and a poor text editor) it is not really an option.
This UNJUSTIFIED NANNYING is more than I can stand. I am too old to put up with this kind of sh!t.