Interesting question. Here's my observations:
The 4-door has 3 rear seat positions. The 2-door has two. The rear seat in the 2-door is much narrower because it is between the rear wheel wells. In the 4-door the rear seat is in front of the wheel wells. So there is a reason the 2-door doesn't have a center belt.
In the 2-door, the center occupant is going to be sitting on top of the buckles for the outboard occupants. And the center belt will require it's own new anchor points outboard of the existing center buckles. So the outboard occupants will be sitting on top of the center occupant's belts. For practicality, I don't see a good solution to carrying 5 occupants safely in the Ty. Take the Type II instead.
As a short-term, short trip, occasional use for kids (small people) I might consider establishing two new anchor points outboard of the center buckles so as to divide the rear seat into 3 sections. Then installing lap belts for all three positions. Use the existing outboard anchor points for the existing 3-point belts. This way you'd have 3 individual belts for the kids, and retain the 3-point belts for use when you don't have 3 rear seat occupants. The center occupant will still be sitting on the buckles, but at least there won't be shoulder harnesses behind him.
:2cents: