The only seats with enough floor space where you can store standard "21 inch" carry ons on the floor are the suites on the B777 and A380's.
I am doubtful you would be able to stow a small back pack under a Y seat, let alone a 21", even if it's a bulkhead row.
Row 26 seats are bulkhead on the B77L. . Cabin crew will happily tell you off and ask you to store it up in the overhead bins as the floor space needs to be kept clear (enforced during take off/landing).