The Artscroll סדורים (prayer books) — "Ashkenaz" and "Sefard" editions, Hebrew-English and Hebrew-only editions, at the morning and evening prayers for holidays, Saturdays, and weekdays — indicate that some congregations say a paragraph starting "יתברך" while the leader stretches out his recitation of pre-"שמע" "ברכו".
I'm relatively young, and have not been to loads and loads of different Ashkenazic synagogues, but have been to a number, including those that pray nusach Ashk'naz and those that pray nusach S'farad and those that pray nusach Ari, chasidic (of various sects) and 'yeshivish' (of various yeshivas, and the yeshivas themselves) and 'yekke' and mizrachi and other, in Israel and the States and elsewhere, and have never AFAIR seen this done.
Has anyone seen it? Or have a reliable report of its being done (nowadays)? Or have a reliable source that indicates that no one does it?