I ran an E-350 (20 W TDP) based board for many years and without the idling drives (each about 0.5 W; only system SSD connected) it consumed about 7 W idle. That's well within your 10 W figure - and boards with lower TDPs are available as well (though a lower TDP does not generally imply lower idle power).

The replacement system (Sandy Bridge i3) pulls about 25 W in the same scenario, which is fine by me.

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Now you're saying that 10 W with desktop hardware is not possible, and yeah, that'll be difficult. But it's a false dichotomy: the "10 W commercial NAS" doesn't have anywhere near that capable hardware. You can absolutely achieve these power consumption levels with similarly spec'd hardware.