1. I asked the poisek Rav Eliyahu Falk in Gateshead if the barchu after maariv is permissible for a latecomer, and he had no problem with it. (obviously that’s not where it’s expressly against the minhag hamokom)
2. Dayan Dunner in London said in his shul (i was there at the time), that there is nothing wrong with it, just that the chazan (for maariv) shouldn’t say it, because then it looks like it’s the shul’s minhag.