Hi guys! good post.
Currently, I've connected my hdd (WD 2TB usb3.0, HFS) in the usb port of my DIR-868L, and I've managed to enter to the hdd through smb. The problem is that router detects a hdd only 200mg, I think that I will have format it in FAT.
I'm going to test this article: http://code.stephenmorley.org/articles/time-machine-on-a-network-drive/
Will keep you informed.