I’m kind of a skeptic on this one. I’ve serviced lavs as a ramper on 737s, A-320’s, DC-9/MD-80. You have to open a little access door and open a valve to empty it. The lav serving port for the DC/MD being below the #1 engine. By the nature of my job, I currently have access to current Boeing technical data, pulling up ATA 38, I’d be surprised to read that these planes can dump while in mid flight.
Disregard, I read a little more thoroughly this time.