Other than in US an Austrian license plate is considered a federal document (like a passport) and has the Federal Eagle stamped on it. This has to do with the compulsory car insurance for which the license plate is proof. If you fail to pay your car insurance then police will take away your license plates.
The license plate is therefore not property of the car owner, and it has to be turned in e.g. when a car is sold or taken out of service.
There is no legal way to keep a license plate. Of course, you can get a license plate, e.g. if it has been (falsely) reported as lost or stolen, or it has been stolen.
I don't know what will happen when your luggage is x-rayed at the airport.
I guess you won't have troubles with real historic license plates, i.e. older than 30 years, when the current white license plates replaced the old black ones.