I'm not as knowledgeable about Chanel as many others in the forum but I've never heard of a Chanel backpack.
I don't think a Chanel backpack is very practical though because backpacks are usually meant to handle some abuse. Also, the pick pocketing factor mentioned before would also make me weary if you're using it to hold your money and putting it on and off to access your things is not very convenient.
Designer handbag yay, backpack nay.