I have rented minivans a number of times over the last few years since I started travelling with kids..

I have yet to get one with captain's chairs yet.. That is not saying that they don't have them, some of the companies probably do. I don't think that any company would ever guarantee you one, as they can't be sure of which van would be at your pickup location at the time you are there.
Your best bet would be to go with a company like National (maybe
Alamo now too) which would let you pick the van you wanted out of what they have available. We went to WDW in June and National had a bunch of Minivans to pick from. Montana's, Ventures, Grand Caravans and Astros. We picked a Montana which had the optional 8 passenger seating. Which was 3 bucket seats in the center row.. Unfortunately Dollar doesn't allow you to select your car.
With Dollar, in many cases, they are giving you a regular wheelbase Caravan and making you pay for an upgrade if you need the Extended Wheelbase Grand Caravan.. I have never seen Captain's Chairs in the Dodge vans at Dollar, but again that does not mean that they don't have some.. I know Dollar has had the Chrysler (luxury) version of the minivan and some of those, if you are lucky enough to be assigned one may have captains chairs..
Just as an aside, I know in the Montana (except for the value package new this year) the Captain's Chairs had been a standard feature in '02 and maybe earlier than that as well. In the Old's Silhouette I think the captains chairs were standard issue as well. We have a Montana at home with the Standard Captain's chairs and we really like them. Good luck and sorry for the long read..
