I dunno how well this will work to bring in the general public's interest as an imitation of the Software Freedom Day model.
With Free Software, you can promote it to the general public by using the "free as in free beer" aspect to hook them in and get them in the door, and then explain the other benefits and advantages. But with Open Hardware there is no "free as in beer", the hardware itself costs pretty much the same irrespective of how its IP is licensed.