I think you'll find it isn't.
These days there are all sorts of regulations about where and how you can fly drones.
For example, only within sight of the operator/pilot, max height ~120m, not within 50m of being above any uninvolved person (such as spectators).
I doubt you'd be permitted to put the pilot in a following car, even open topped, and anyway, the last thing needed is yet more cars.
Those races I've seen that have had drone coverage (MTB & CX mostly, though there has been the occasional road race), have all limited coverage to relatively short sections of spectator-free route (up to half a mile/800m), which isn't too bad when it's laps of a circuit, but just seeing the peloton whizz past once wouldn't be worth while in most road races.