I went ahead and made it, only took a couple of minutes... 
Solder-Mask-Keepout_12mm.fzpz (2.1 KB)
I put a silkscreen around it so you can see where it is at, otherwise it would be invisible. You can remove the silkscreen by right click, Hide part silkscreen. It checks out on OSH Park gerber test...
Please let me know how it works when the boards come back.. Post a picture...