(i) How to de-register a car from State registration?
(ii) How do you provide notice to the US, State of California or Canada concerning the uniqueness of your plate/tag?
(iii) What kind of information do you display on a tag?
(iv) What suffices as proof of right of possession other than a Certificate of Title?
i. You can transfer it from domestic to not domestic and/or from public/residential to private using by gifting/selling/assigning. States issue CoT to *residents*. Foreigners can be lienholders. Oh believe me I've had one on one with top-level DMV heads of more than one State. You find a "notice of sale" form and make a generic one using that kind of information and mail it to the DMV associate with the Title Document.
ii. You could send a notice to State Patrol, Attorney General, Sherriff, Chief of Police about the unique tag. Depends on where you are "coming from" so to speak. In the box? Out of the box?
iii. If identifying the particular automobile is key then the VIN # or the last eight digits (CA/CO) might suffice. Does your name go at the top of the tag? What goes at the top of the tag? Does the name of the assignee go on top of the tag? What's the significance of using the State's name vs the name of an assignee on a tag? 2" to 3" tall lettering with high contrast between letters and background tends to be recommended for automobile signage/decals.
iv. An MSO. Bill of Sale, Gifting or Assignment + MSO. Mechanic's Lien form. Other Lien form. Junk or Salvage Title. Notice of sale form.
http://www.odot.state.or.us/forms/dmv/6890.pdf
http://www.flhsmv.gov/dmv/forms/btr/82050.pdf
Those are examples of how you could make your custom notice using a Word Processor.