I am not a lawyer, but I do not believe that DKIM provides repudiation specific to an individual. DKIM provides evidence that email originated on an email provider. The users neither own nor control the server and user accounts get compromised all the time as well as fake accounts created all the time. Google battles this daily. DKIM might be one piece of information, used in combination with a client IP address and some method of proving who was at that client IP address at the time, but I do not believe that DKIM could stand on its own.

For example, I have servers that DKIM sign emails. If a person uses my servers to send a death threat, the FBI is going to want web access logs and smtp logs.