How should a reviewer provide comments to a journal's special issue in a field he is not familiar with?
I have been receiving a lot of invitations lately to review proposals for special editions -- mostly open access.  These proposals have been way outside of my area of study (and sometimes discipline).  My guess is they scrape the web for potential reviewers and use a shotgun approach -- but that is just a guess.  

I ALWAYS decline serving as a reviewer if I am not sufficiently expert on the substantive or methodological issues.  I also try to highlight in my review where my expertise may fall short so the editor knows where to fill the gaps.  

I suggest you decline any invitation if you are not qualified to review the topic.