If that contractor says that it will not allow attic access, it's probably because the snap on piece is a simple trim ring, meant to hide the rough edges of the siding, and the louvers are attached to the nailed on section. I've seen that--trim rings to hide edges only--before, although not on the type gable vent you describe, which I haven't seen.
Now it makes sense what the guy told me: only the trim "ring" will come off, not the center piece. He mentioned the rough edges of the siding and I couldn't understand why this would still not give access to the attic.
BTW, "access" was only for my left (or right) arm, not my whole self.
So now it is going to be kneepads on, nose covered, and then: no venture, no (antenna) gain. Ugh!