I've thought of doing this before, essentially "meadifying" one of my beer recipes (a chocolate cinnamon chipotle stout). Usfbmike is correct...it does take a LONG time for cocoa to settle out (and ultimately, it helps to do some finings) but it can be done, and I've made a straight cocoa mead before with powder successfully. That being said, it takes quite a long time for cocoa flavor to incorporate properly in a mead, and it also requires a little residual sweetness to come out right (otherwise it's just an odd sort of bitterness), so you might need to backsweeten to taste (I left mine at mid-semi sweet, ~ 1.014 after backsweetening).
I'm working on a cocoa bochet right now, and after I see how that one turns out, I may start another cocoa mead with cinnamon and chipotles, +/- caramelized honey vs plain. One of these days I'll try the nibs for comparison...