Vehicles without reg can be towed legally as long as the towing vehicle is up to date… at least if it’s on a trailer or 2-wheel tow dolly, and depending on state. Writing “in tow” on it is a common thing to do if they’re being flat towed (aka not on a trailer but via a cable or tow bar) but there is no legal protection provided by it; since the vehicle is entirely on the ground it needs registration basically everywhere AFAIK.

Dealers and some mechanics shops get assigned dealer plates that they can affix magnetically to any car to indicate ownership for transport or test drives.