To me, these look like the closest edge-cases yet. But is Humpty Dumpty also a middle pop? I see flippers on the sides, I see a bumper in the middle... has to be one?
Register also has backwards flippers and a bumper in the middle. Middle pop?
I didn't consider either one because Humpty's layout was less of a deliberate middle-pop design and more a matter of covering the playfield with them newfangled flipper contraptions and seeing what happens kind of thing. Yes, there are technically "pops in the middle", but it's more a matter of circumstance. Reverse-flippers play differently than most games and have different ways of interacting with the geometry. As for Register, it has a dead pop like Safari which disqualified it.
In Judy's case, it has a cluster of dead rebound posts in the middle... like a dead pop. As such, the lower pop is the only zone where any action would take place. Considering the flippers are backward (and most upward kicks would be blocked by the post zone) they'd hardly be able to interact with the lower active pop. Which means the design wasn't intended for that kind of interaction. And, if not, it wouldn't make the cut in my estimation. Looks more "Humpty" than "Rose Bowl" here.
Oasis gets us a bit closer. Pretty much the exact same design as Judy except the pop bumper is active, in the right place, and can interact with the flippers. However, again the layout strikes me more as a circumstantial holdover from the Humpty-era than not. But still... a tougher call on this one. Clearly some deliberate thought was applied to the flipper/pop interaction in the middle. Hmmmmmm... need to kick this one around for a bit.
If not a hard yes then certainly an inspirational one!
Thus far, the Marchant you found is the earliest obvious pick. Deliberately designed to be a middle-pop, no question.