Many thanks to faithful reader "LGR," who brings our attention to a 1981 photograph of the Flatbush Pavilion (a.k.a. Cinema Plaza):
It is instructive to note the condition of the marquee in 1981, above, compared to the recent views below. All the detailing seems to have been shaved off:
"LGR" also asks: "Does anyone happen to know where on Flatbush the mezzanine exit from the subway actually let? I simply cannot recall from my hazy youth, but it must have been around here."
Our understanding is that the mezzanine exit was through the closed door with the ornate surround, between Antonio's Pizzeria and American Apparel, shown below. The Park Slope Civic Council has been exploring the possibility of reopening this closed entrance to the 7th Avenue station:
Note that in the 1981 photograph at the top, the door seems still to have been in use. Also note that the classic Antonio's Pizzeria sign seems to be unchanged since 1981!
The StationReporter description for the 7th Avenue station notes that there is an emergency exit at the extreme southern end of the southbound platform. Thus one would expect to find an unmarked door, somewhere on the west side of Flatbush, to the south of the current station entrance. A possible candidate might be the white door with no knob, below: