USB-to-Ethernet works fine already, you need to make sure you use an adapter that works on 'linux' in general, and it will work fine on the DB410c. I use one everyday..
There is not Ethernet controller on the SoC, or on the board, so if you want to add Ethernet functionality you need to look at the interfaces available on the expansion connectors.
One option is to use SPI. We've been able to hook up and ENC28J60 based SPI Ethernet controller on the DB410c, this one. Ethernet over SPI won't give you fast ethernet though.. but depending on your needs that might be enough.
Some folks at Linaro are trying to design a small debug board which has this SPI Ethernet controller, so potentially that could be an off-the-shelf mezzanine board that anyone could use..
You don't really need the schematics to do that, since you can directly use the signals from the low speed expansion connector.
I don't know about Android, and how it would handle Ethernet connection, though..