Does the D-Link DIR-857 support streaming Time Machine backups from multiple machines?
Product description
According to the datasheet, it looks like this device's share functionality does not work with Macs at all, since you need to install a driver to get USB-via-Ethernet support. From the footer of the last page of the PDF you link to:

Minimum System Requirements
Computer with:

Windows® 76, Windows Vista®6, Windows® XP
SP26, Windows® 20006, or Mac OS® X (v10.4)5 + Internet Explorer® v6 or Mozilla® Firefox® v3.0


And 5 translates to:

The software included with this product is not Mac-compatible.

SharePort drivers
There are SharePort drivers for OS X though:

This forum page links to a driver download for OS X up to 10.6.
This manual (zipped PDF)  is linked as Additional downloads and also mentions support for OS X up to 10.6.

This is for the &quot;USB via network&quot; feature, so this might work. The advantage is that the file system on the drive should be transparent to the router just forwarding the USB connection. That's why they can say they support any USB device: They don't actually care what kind of device is connected.
Regular sharing
It doesn't look like this device has any regular sharing functionality. Wouldn't surprise me, given the networked USB feature.