Starting the following questline will eventually turn it repeatable. Doing turn ins for this quest will give you rep with Darnassus.
http://wow.allakhazam.com/db/quest.html?wquest=3790
Additionally, this questline also gives you Darnassus rep. It also becomes repeatable.
http://wow.allakhazam.com/db/quest.html?wquest=7792
Good luck.