I talked to Freedman Seating about getting retrofittable bus seats that had LATCH anchors and 3 point belts. They have a model called the Family 3PT that is reasonably customizable and is actually made for school busses.

It took some back and forth to get Freedman to agree to sell me the seats; they are pretty concerned about their liability and the fact that the mounting system was not something they had personally tested.

View from front of bus looking rearward.  Seats and car seats in the foreground

You can see in this photo that two rows of seats are installed, with a nice center aisle. The seat units are 35" wide pairs that mount to a common frame and adjustable leg assembly. Each position has an independent 3PT belt and complete 3 point LATCH system (2 lower anchors, and one tether connector around the back side of the seat)

As you can see, our twins, who were rear-facing at the time, can sit side by side on the same seating unit. Loading them in and out of their car seats was very, very nice, since we got to stand fully upright, right behind the seats while installing the children.

Here is a youtube video that shows an in-progress walkthrough of the interior, and spends some time discussing the seats.

Our older son's booster seat we moved around from time to time. My wife sat directly behind me so that we could chat more easily and so that she could manage drinks and snacks for me. I did all of the driving on our trip.

Freedman offers some cool options, like built-in AC and DC outlets on the seat frames. However, the options add up and we spent more on the seats then we spent on the bus, so we made due with adding extension cords to nearby wall-mounted outlets.

As mentioned on the building page, the seats are mounted to L-Track rails that I installed in the front of the bus specifically for seating.