I think this is a really good theory. Or at least Rock might have been a Dawnshard at one time (or had some Connection to one).
A few other points that support this:
*Rock seems to have a very well attuned sense of taste and musical pitch (his humming in WOK when they first enter the chasms). Rsyn similarly gained some heightening like this after she became Change. (Although maybe this is just from the Peaks' investiture pools.)
*It looks like the Dawnshard's effects continue after someone had/was one (like for Sigzil and Hoid, who still can't eat meat even after giving up the DS).
*We know a Dawnshard has come through the Peaks at least once (albeit apparently long ago, but maybe the effects continue or tie to people who have any type of Connection to it).
*Hoid's fight with Kelsior shows that the no-harm effect isn't categorical like Yuliya said (and,if Dalinar is a Dawnshard, we might see the same when he fights in WOK but then gets unnaturally sick about it).
*The Change Dawnshard seems to be self-aware, and it seems to like Cord for some reason (it pulses with warmth when Rsyn lets Cord keep the Shardplate).
*Rock is named after a special rock that his father saw, and it looks like Hoid's prior Dawnshard had some tie to a rock and/or Topaz.
*The Sleepless seem to have an unusual respect for Horneaters.
My best guess would be that Rock either was/transported a Dawnshard for a short time, or the Dawnshard's effects tie to anyone who is significantly Connected to another person who was a Dawnshard (like maybe an ancestor of Rock).
