Perhaps a better way to do this...

Problem: we want to make a negotiable instrument ownership of which is identified cryptographically, but it might be legally problematic:
1. we don't know whether court system will recognize cryptographically signed transfers of ownership among anonymous parties
2. it might be illegal to make such instrument because it is seen as security, which is subject to regulations

Possible solution: disguise this negotiable instrument as simple non-negotiable person-to-person loan. (Which likely is legal.)

Imagine a contract where one party's name and signature is left blank, but it is signed with other party. Whoever has access to this contract physically can write his name and signature and effectively becomes a payee.

To do this, we'll make contract with two or three parts which are signed independently. First part will describe issuer's identity and general terms of payment. Second part will describe creditor's identity. Third part will be a letter from creditor which will say that he wants his money back.

First and second parts are signed with issuer's digital signature. However, it is a lie: issuer will reveal his private key to a 3rd party, which would act as a notary.

Notary will receive a request from creditor, check whether it is valid and then he'll fill second part of contract with creditor's information and will sign it with issuer's private key.

But a first part of contract will state that both parts are signed by issuer personally at the same time so they form a whole valid contract.

So how will we prevent abuse from notary's side? Contract will also include a clause that to claim money payee will have to identify himself via a certain chain-of-digital signatures scheme. So that's how we connect decentralized market crypto scheme with a world of two-party contracts.

E.g. "I'm John Smith. Please pay me to this bank account: xxx. My digital signature: yyy. A chain of digital signatures which confirms this request: zzz."

Contract can explain that this weird chain-of-signatures is requires to prevent identity theft or something like that. It's even partially true...

Additionally this scheme can be enhanced with mandatory arbitration clause in contract. The reason for this is that it's likely easier to find a tech savvy arbitrator who will understand how to verify a chain of digital signatures which controls ownership then it is to find a competent court. Arbitrator will simply say that he have verified all this digital mumbo jumbo and court will recognize it.

Additionally, this would guard issuer from potential abuse from notary as such attempts will be filtered out by arbitrator.