ERISA section 3(39) does not seem to preclude this definition.
Review Rev. Ruling 81-159, which addressed a 52/53 week plan year. It may be possible to define the plan year using the last paydate, but this RR implies it must apply for all purposes (vesting service, etc); unlikely that the sponsor could use that definition unless it's in the plan.
I might want a letter ruling on it, but that's just me.