I found the control that loads all the meta tags from the Kentico Portal Template. The control ID is called tags, it is loaded in PortalTemplate.aspx. Found og:title tag is in a string in the property Text.
Since the tags are loaded into a System.Web.UI.WebControls.Literal, I guess Kentico PortalTemplate does support editing a single tag by looping the HtmlMeta controls to find the tag you want to edit.
I am now looking at editing those tags in OnPreRender now.