> I want less than 10W idle for the whole system, maybe except HDDs, but even those will be in sleep much of the time. x86 boards are mostly ATX-powered and I don't think any ATX power source can go that low and still be efficient (not draw 20W while powering a 10W system).

If you look at some industrial boards (e.g. from ASRock) they're DC-powered. I haven't actually measured the power draw on mine - I'll try to remember to do so next time I power cycle it.