It feels a bit like "cheating" that new x-string features are built-in only. It would be cool to be able to do:
    from foo import bar
    bar"zoop" replies(7): 
    from foo import bar
    bar"zoop"  sql"..."
  html"..."
If you pass a "t-string" to a framework, it can force escaping.
What you suggest is to rely on escaping by the user (dev), who, if he was aware, would already escape.
Unless you'd suggest that it would still return a template, but tagged with a language.