Hmmm.... this is a great question! I think no matter what you do, the weak point is going to be the backpack itself. I seriously doubt it's designed to hold up to crash forces, and they may not even be designed to hold more than the few pounds that you normally carry in it. So... my thought was to find ways to make the backpack straps take as little crash force as possible. I'm not sure if this would work for you or not, but attaching it to the back of the captains chair that your DS is on could accomplish that. In a frontal crash, the vehicle seat will take most of the force of the impact, putting very little on the straps. In a rear crash, the straps would take the crash forces, but rear impacts are usually less severe and the forces would be smaller. (It's the same reasoning for keeping a child RF.
) In the event the straps break the the backpack goes flying, hopefully the 3rd row of seats would keep it from going so far that the feeding tube is ripped out.
I have a couple ideas on how to attach it to the back of the seat. I'll explain briefly, but a picture's worth 1K words, so I took a few
These might not work for you, but maybe they'll give you some more ideas.
If the backpack straps are long enough, you could hook the straps around the armrests on the captains chair. Lenghen the straps, flip the armrests up, slide each strap over an armrest, then flip the armrests down. There's a possibility that the armrests could flip up in a rollover and the backpack slip off, but I think that's pretty unlikely.
Since you have your DS's seat installed with LATCH (I think?) you could secure the backpack with the seatbelt. Thread it around the back of the seat, through the straps, and buckle.
Use some kind of cargo tie down strap and wrap it around the seatback and through the backpack straps. It would be best to use a heavy duty strap that's designed to hold very large loads, preferably with a fastener that's easy to hook and unhook. (the strap in the picture is not a load bearing strap, but it gives you the idea)