It's already part of Bitcoin: You'd simply assign your coins to a 2 of 2 multisignature with your escrow service. It's implemented in practice too, in varrious

It's already part of Bitcoin: You'd simply assign your coins to a 2 of 2 multisignature with your escrow service. It's implemented in practice too, in varrious forms including ones with timeouts in case the escrow service goes down or tries to extort you itself. It turns out people mostly don't want this.