You can write it on the empennage with a Sharpie and be perfectly legal.
I did it myself when the mechanic wanted an arm and a leg for a metal plate.



One thing I noticed was that the "normal" engraved metal manufacturer's
data
plate that's usually located under the horizontal stabilizer is missing.
In
its place is an engraved plastic plate with the serial number, model
number,
etc - the kind of engraved plastic you think of for office nameplates.

Is this legal? Warning flags? Is this the *only* location of official
serial number documentation?

Thanks;
Carl