A transaction can be written that requires two signatures to spend it next. You write a payment that requires the signature of both the recipient and the sender to spend it. To release the escrow, you give the recipient the signature for your half, or the payee can return it by giving you his signed half. There's no mediator in this simple case. The recourse is to refuse to ever release it, essentially burning the money.
Due to that recourse, it is unlikely to be used as an escrow mechanismReally? Do you think people won't be able to understand the benefit? (If your response is an argument that there's no benefit at all, I guess that will reinforce the case that people won't be able to understand it.)