In the middle row I have only the two captain seats.
However, one time I removed the captain seats and tried placing the third row seat/s in the middle section and I was able to latch them in the middle row. I have also had the intention to purchase the third row seat from a junkyard.
For a reason, the owner's manual or in the glove compartment box, the manufacturer only recommends to leave the seating as it is originally configured... check it out.
But it would be neat to seat 8 passengers in my case since I sometimes have other people ride our van when they need a ride.
I don't see any reason why not.. Ingress / egress might be a bit rough for those in the rear having to climb over the (folded down) seat, but the rear seat is designed to fit in the middle for times when you need the combo of seating and cargo space in back.
We bought a used rear bench off of a craigslisting page and are sporting a 2003 ES 8 passenger van. we have six children and so we improvised. The rear bench is made to fit i the middle position so we figured it mounts oem, the seatbelts are oem , why not go with it ...oem.
The only issue I've had with the rear bench being used in the middle row, is that it's narrower than it should be. And without arm rests on the outside, I found adult passengers (ie: not in anchored child seats) slid around a bit. As far as safety goes, that was my only concern.
Another minor issue with it, is that the rear bench only has a spot to ancher a single child seat, where each of the captain seats had their own anchor points.