Responding to borchu basra according to minhag Ashkenaz

According to our מנהג:
אין אומרים 'ברכו' בתרא אחר התפילה, מפני שאין ברכה אחריה

May one respond to one reciting ברכו after תפילה? Is there any difference, if there is just a מנין with him (exactly ten people answering)?

Rallis

There is a common conception (or misconception), that if one walked into schul late for marriv and missed barchu he may recite barchu after aleinu. Now, the halacha is that if one did not say birchas krias shema, he may recite the barchu as it is going on the teffilos he is about to say. However, are there Ashkenaz sources for a person who is totally done davening to just randomly say a barchu?? Even if there are, I am willing to bet it is not a German custom.