It is possible now! With Facebook Chat Auto Reply v1.0 (online application), you can easily set auto-reply for your facebook chat.
And, it is Free now! So hurry up and sign up today!
Instructions to setup Auto Reply
Some features of Facebook Chat Auto Reply v1.0 :
Set auto-reply timer
Set up with one click:
(below is an image, Click instruction to setup)
Custom reply message
More features coming soon!