I guess it would be easier to set up with two key ingredients.
First, finished projects. No risk in some of the collabs to drop from sight and leaving the project before it is done. You have a sellable item and can concentrate on selling it.
Second, take a third of the sale price. This will take care of lots of services that include dealing with additional requirements for revenue shared projects. Including being the merchant, instead of the individual collabs being the merchant. If all those pesky details are dealt with, it is only a matter of paying out to different accounts.
Maybe the thing with the merchant is the biggest hurdle for itch to implement such a thing. They have the option to not be the merchant for an item, sometimes they even require it. Bottom line, that status can change. But it could not change for a revenue sharing project. How would that work, if a user wants to buy a thing from 10 different merchants at once? From user perspective it has to be a single merchant.