#[2] just had a poll asking what the next feature should be on Amethyst (He's the author), and the winner is the marketplace feature. Payment splitting is probably not part of the NIP, but you might want to mention it to him since it looks like he's about to do the first implementation.
Discussion
Now I think about it, payment splitting for products might be more appropriate under bolt12, where the routing is specified in the invoice rather in the marketplace. I highly doubt the seller wants the splits to be known by the buyer.