I am looking to do the same thing - use a tablet instead of the Squeezebox (too slow) and connect it via RJ45 (we are also a WiFi-free home). I understand that connecting with a USB-to-Ethernet adapter is possible. But that leaves only the headphone jack or the HDMI connector for the audio out. Is it possible to get the digital audio output from the HDMI or headphone connector? Did you manage to do it?
If you are planning on leaving it in a single location, you could simply connect it up to a USB hub.
Many devices these days have a combined 3.5mm analog/optical connection, so it may be possible to get digital audio out through that.
HDMI also supports digital audio, but you would need an HDMI-equipped receiver for that.
The Gigabyte tablet that was originally linked to here is fairly unique, in that it actually has an ethernet port on the side.
Microsoft is also due to announce the revision of their Surface line today, which is rumored to now have an optional dock, that may simplify connecting multiple devices, and look neater if the cables all connect to the back.