How to run a Desktop graphical application just once under sudo?

Solution 1:

You can run the software from the command-line/terminal with gksudo prefixed to the command, which will present the prompt for your sudoer's password prior to launching.

gksudo file-roller

Though you mentioned you do not want to make this a permanent change - you could add a new Menu item into something like Applications -> System Tools via alacarte (System -> Preferences -> Main Menu) The command for which could be gksudo file-roller creating both a regular Archive Manager launcher and a "root" Archive Manager launcher - for convenience.

Solution 2:

Check out the package nautilus-gksu. After installing and restarting your session (or just nautilus) you'll be able to right-click on any file/folder and select "Open as administrator" and be prompted for your password. Works well for opening root nautilus windows, root archive managers, root text editors... you get the idea.