There is a new control unit that can possibly replace your 'Wagon' portion of the Disklavier. Search at Yamaha.com for the 'DKC-850' control unit and check the specifications. It replaces many control units, and can be used through some older ones. Not sure about the 'ol Wagon grand.
You can certainly obtain a service manual for the Disklavier, which will show components, locations, and service requirements, but the actual schematics??? Not sure about that, sir.
The Yamaha.com website can get you to 'Support' where you can order the service manual (...you'll need the model number) or you can just go to 'Contact Us' on the same page and shoot them a note. Explain your quest and see what happens! I have never waited longer than 24 hours for a response.
Another option is a phone call; 1-800-854-1569 (Yamaha Service)
But, there may be a bit of a wait, and you have to work through a 'phone tree' to get to the right department. You'll want 'Acoustic Pianos', and then 'Disklavier', and then wait a while for someone to get to you. I think you'd do better to start with an eMail, as you can better explain your needs, and give them the info you're after more simply.
Yamaha supports their products quite well. I've gotten parts for keyboards built in the mid-80's, complete with instructions and diagrams for installation. Good folks at the service dep't, and they should be able to answer your questions and needs.
Hope this helps!
Jeffrey T. Hickey, RPT
Oregon Coast Piano Services