I think we'd probably better off with the previous idea: just work for them for a period.

I'm speaking from the perspective of company A, who needs a feature added to company B's product.

They could send their engineers to work for company B, sure, but those engineers' time is still costing money. And those engineers are completely unfamiliar with B's codebase, so they won't work as efficiently. Might as well just pay company B directly for the feature work.