As far as I've ever seen, Apple has never published specifications for the iPad pogo pin connectors, which is why you've only ever seen a handful of Apple-blessed third party accessories. Which is a shame and part of the laundry list of ways Apple kneecaps the otherwise extraordinary iPad hardware. Would love to see what more nimble third parties could do with that connectivity that Apple hasn't. So unfortunately, the only chance you'll have is to find someone brave/determined enough to have reverse engineered the signaling in order to market a product. If such a thing exists, it'd be on AliExpress I bet.