I would imagine that this may require the licence legal text to be rewritten, but not much else would change. Instead of selling a license to use, you sell some sort of authentication service. So that when the actual right to use is transferred, the new owner would need to pay for the authentication service.
As for Valve, they are moving strongly to the free-to-play-but-buy-hats model anyway, where you already can give away all your hats to someone else. Although, yes, it could impact other sellers on steam.
Instead of selling a license to use, you sell some sort of authentication service. So that when the actual right to use is transferred, the new owner would need to pay for the authentication service.
I sorta feel/hope such clear attempts to avoid the law by trying some sort of Jedi hand wave would not pass muster with the courts.
As for Valve, they are moving strongly to the free-to-play-but-buy-hats model anyway, where you already can give away all your hats to someone else. Although, yes, it could impact other sellers on steam.