There is one weak point here: you have to trust that the owner of a correctly-attributed bond key will actually construct a valid (protocol wise) bond transaction that transfers control to the owner_pubkey you requested. Given that you're also trusting this person to repay the bond, this is not necessarily a big deal, but it'd be nice to require it.
You have to trust the issuer of a bond to repay it, but you should not have to trust the previous (or any other) owner in any capacity.
This seems like a fair criticism. There are two parts to a typical bond transfer,
1) Owner #10 must create and sign a well formed BOND message to Owner #11
2) Owner #11 must create and sign bitcoins to Owner #10
Each party is holding something of value, and wants to trade for the other thing. What does game theory say?
Each party's action may be instantly validated, but who goes first?At this juncture, perhaps some readers may be directed to the just-created Atomic Coin Swapping thread, if they have helpful ideas.