If a user uses escrow and the escrow goes well, then you rate the user for 0 risked BTC and the escrow agent for x risked BTC, both positively...
Right now escrows are leaving positive trust feedbacks for people just for using their services. So that's probably a great idea.
People can't just get positive feedback because they have done a smooth deal through an escrow, IMO.
If I were an escrow agent, I probably wouldn't ever leave positive trust, only a negative on the scammer's side (the thing I'm escrowing does not meet the terms of contract) - or a neutral/suspect (one side I suspect to be a scammer, but the escrow is completed with merely suspicious behavior over the time elapsed, or suspiciously fails prematurely, like during the pre-delivery phase).