It looks like the seats are separate parts from the undercarriage, so he has the option of making it a magnetic connector with additional taller seats. That might add too much cost, though - I just don't know.
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View attachment 505982
Better still, he could make it really cost effective by simply just adding a removable fitted "booster" cushion on the top of the existing seat bottoms that could be taken out when/if you want to add the figures. That would work just fine, but I'd gladly pay an extra charge for them if need be.
He could make any of these options an "a la carte" item and charge for the seat upgrade, just like the bvs mobile and the additional justice league kit.
Here's hoping!