Yup! I’ve done it many times. We have to be seated in at least E+ if we’re in the back. Regular economy we can refuse. If we take the illegal Econ seat we can get a credit to use for a first class upgrade when we normally wouldn’t be entitled to it (DH deviation or some training travel). If we are in an assigned middle seat (we can’t move ourselves there) we get a 50% override for the scheduled flight time as add pay. Honestly the only change I’d make is the ability to refuse the middle seat assignment or take the add pay. I did a 4 hour middle seat DH not long ago. It was torture. But as long as the middle seat is in E+ it’s legal.