I'd have to look at it, but couldn't you just put something that spans the center aisle from the helm seat cabinet to the dinette seat cabinet. Certainly if you have the forward facing dinette seat it should work. Just get a board that spans the gap and screw a seat to it. The board and seat would also be removable if you didn't need it.
When I travel long distances I take a break from sitting by standing in the aisle and steering with my right hand. If I had kids on board who wanted to sit up there, I'd let them sit in the driver's seat and I'd stand next to them in the middle.