Believe so, mainly. SIS cable pull should be the same. A little adjustment of the limit screw and I imagine you'd be good to go.

Then again, I feel like I've heard something about a slight change in the angle of the parallelogram between 6sp and 7sp Shimano designs - but I don't have a source for that offhand, and I'm not sure if it would have any noticeable effect on indexing performance if true...I've used 6sp 105 RDs on 7 and 8, for sure, and 7/8 Shimano RDs on 6, but typically in friction mode. So I'm no help with definitive experience regarding possible indexing issues. I tend to think it might not be too significant a hitch, assuming I'm not just making that up in the first place - and a coop find RD seems like it'd be worth just giving it a try, but I'd defer to others with actual experience (and more than just a vague recollection of the facts) on that one.