Asking a complete stranger for a subreview for a conference?
I am a CS guy and can answer this question. 

Yes of course. That is what conference chairs do perhaps. As you told the person has already published in past conferences like this. If some researcher is active in the field of his/her research arena and agreeing to review the manuscript. This is what is called peer-review. My recommendation would be to contact the researchers through academic e-mail ids of theirs to make it more transparent.