Good idea BUT problem would be that to be useful it would need to be adaptable for other uses easily.
Nobody wishes to carry around an IPAD device along with everything else just for a plane therefore your target market may not be users but airlines.
Problem then with airlines is that they would need to charge for it and ensure if plane sets off with 25 from LHR they will get 25 returned and reusable in LAX.
Its a nice accessory BUT question is how much would people pay for it and could you sell enough volume to justify its development.