You could always run the command in a terminal.
Your notification is when the terminal closes itself :). It will also show whatever progress / activity indicator is provided by the unar command.
gnome-terminal -x unar -- %N
I have not tested whether xfce4-terminal accepts the -x option.
xterm -e unar -- %N
uxrvt should also accept the -e option.
Apologies in advance for any eyestrain due to running xterm with its default font size.
gnome-terminal also has a -e option. With gnome-terminal, the option takes a single command argument, and splits in based on spaces. E.g. gnome-terminal -e "sleep 1". We can't use this because filenames could also contain spaces. With xterm, -e can actually behave either way, depending on how many arguments you pass. So the behaviour of gnome-terminal is less magic and probably nicer, provided you don't mind that gnome-terminal --help fails to document either option.