Shaqd

I have done this, although I am not familiar with the canonical method you mentioned. Social share counts are tied directly to a URL so if the structure of a URL changes at all the old count will be attributed with the shares. You can create a custom social share button that pulls the data from the old pre  url structure change. Mike King has an awesome post about how to do that here (which is what I believe you are referring to with the og: url propterty?): http://searchenginewatch.com/article/2172926/How-to-Maintain-Social-Shares-After-a-Site-Migration 

You can read a thread where he and I discussed it here: http://moz.com/community/q/preserving-social-shares-through-url-changes

His method works.