Maybe not the Y+ option, but isn't this precisely what Qantas tried first with Australian Airlines (the ocre version!) and their all-Y 767 services ... which failed ... before looking at JQ?
Regards,
BD
Australian Airlines didn't fail. Australian was killed off because it was easier for Qantas to use Impulse to form Jetstar rather than expand Australian. So it was redundant. In many ways Qantas' experience with Australian is what has helped make Jetstar a success.