shlishi –
“Does Reb Moshe’s psak prohibit having a private eruv in one’s own driveway and backyard in Flatbush or Boro Park (like many people do for Succos or year-round)? What halachic reason, if any, would such an eruv be different than a neighborhood eruv? And where would a block-wide eruv fit in according to Reb Moshe?”
Driveway and backyard eruvin are part of a reshus hayachid. No one takes issue with them. However, block eruvin would not be allowed according to those who argue that Rav Moshe would not allow an eruvin in Brooklyn.