I'm aware that there's no field in the record format that explicitly relates to co-ownership.
This scenario must come up fairly often--one party owns (is the registrant) the domain name, another party actually operates the Site. In my case: my employer is about to license an e-commerce Site to another outfit. My employer is the current registrant of the domain name.
Obviously, the licensee should have access to the domain name record to re-point the domain name to their own web host, to add/change nameservers, etc., change the admin contact address, change the entire MX record, etc. Still, the licensee is just a licensee not the actual owner/registrant. That's important because for instance if the licensee breaches the license agreement, for instance by selling our competitors' products, my employer has the right to terminate the agreement--but that's not worth much if they can't regain control of the domain name.
A couple of ideas i've had so far:
rely on the 'two-tired' account authority which our Registrar offers (which most others likely do as well), i.e., retain myself as the account's superuser, add the licensee as an admin on the account, so they have full 'working' access to the record;
lock the domain to prevent its transfer by the licensee ('CLIENT TRANSFER PROHIBITED' under the 'status' field), though perhaps with admin access they can change this.
On several occasions, we have purchased domain names from 'professional domain name owners' and used the escrow service provided by our Registrar, and it's worked very well. The intention behind such services is obviously to protect buyer and seller during transaction. A service directed to license-type transactions rather than outright changes of ownership would obviously have to function quite differently, still i thought perhaps such a service might offered by one or more Registrars.
And if you had in mind to reply "that's a legal problem", please don't. It's not a legal problem or a sys admin problem, it's a business problem. And like many business problems it has multiple possible solutions from various functions, legal, IT, and whatnot. The legal solution is embodied in the License Agreement. The Sys Admin solution is directed teh administrative minutiae of the domain name record in order to reconciling the possibly conflicting interests of two parties both of whom (apparently) need access to the same domain name record for different reasons.