I guess the closest thing we have here in the US would be Consulting, like Ernst & Young, or something.

But that is a VERY different corporate relationship/agreement than a secondment.

It seems to me that a secondment is based sort of on a gentleman's agreement that both companies will benefit from this in the long term, and so - while the exact dollar value of the benefit is unknown - both companies agree the benefit is there. So they do the deal, and share the resource (person/skills).

Competitors used to do this more often; they'd be honest about a common wall that both companies faced - like, say, better data compression would help both companies' products, but neither had the full resources to crack the problem - so they would combine forces to innovate a solution, and would both share in the results of the research.

Xerox PARC was based on similar principles... smart people from all different companies and backgrounds coming together to jointly innovate...

Hm. Secondment. It would take a VERY different kind of American business to embrace such a thing. They'd be too defensive to collaborate/share in any significant way.