You know far more about these cars than we do, so we're more stumped than you. We found another single bracket like this among a box of parts taken off a junkyard Firebird coupe years ago. This one has no paint on it at all.
It's either a rod guide as you suspected or a "hook" for interior trim, judging by the design. You can see marks in the pictures on one side (the bracket on the right side of the picture above) of the small hole where the screw head contacted the bracket.
