See #32: perfectly legal to put the ID on it where it cannot be seen (suggested inside the battery compartment). Ergo, lack of immediately visible identification is legal until FAA has physical possession of the drone - eliminating most of the justification for having an ID on the device in the first place, leaving the only use of registration being after accident/crime where the drone has been acquired by authorities, or some actionable use identified AND the operator located.
I have already decided to post the FAA registration number in encrypted binary in a hidden data area of the flight controller firmware. It will be 100% compliant with requirements for visibility and legibility if you know where to look and have the decryption key or access to NSA resources.
Where there is room I already display my old plane’s N-Number on model fuselages.