I’d love to see how you accomplish this. It seems tricky to be able to sign the replacement transaction without revealing the encrypted private key either to yourselves, or to the mobile app. I guess that you could have the app send the encrypted key up to your servers, where it could be decrypted locally for use and then discarded. But if you are going to do that every time the note changes hands, why not just hold onto the private key? Why mess with encryption keys?
You might just need to bite the bullet and maintain custody of the secondary key server side. Is there a particular jurisdiction you are worried about that is motivating you not to want to maintain custody of the secondary key? In the US at least my recollection is that you are a cryptocurrency custodian only if you control all the elements necessary to transfer the bitcoin—meaning you wouldn’t be a custodian because you wouldn’t have access to the user key. I may be wrong about this, though.