I have manage to reach 14W idle on a 2100T and intel mobo (but no perif), the setup is on my signature, it run 3 fans, i bet with one i can get close to 12 or so. When i was building my server, i stumble on a CPU that i always wanted to try, but never had the chance as it was only OEM from what i remember, E3-1220L its very similar to 2100T but had a lower TDP, but was long time ago, dont take my word for it, as i never really tried it.
Maybe it would be worth to check Intel DN2800MT hands on...Single digit world?
since you already own one.
I bet with mobile chipset and ULV mobile cpu you might be able to get below 10W, maybe with haswell.... who knows.