"How this would be handled with block signaling is anyone's guess. That's above my paygrade."
In most signaled territory, whether ABS rule "251" (where trains run in a specific direction on specific tracks) or CTC rule "261" (where trains run in either direction on a particular track) the block signals "supersede" the authority of extra v. regular (scheduled) trains.
In other words, all you need is the signal (from dispatcher, or operator where authorized by the dispatcher) to enter the main track. Once you're on the main track you just run as would all the other trains, governed by block signal indications.
Some early block signal installations did not go this far.
All they showed you was the condition of the immediate track ahead.
In this territory, trains still needed timetable or train order authority to occupy the main track.
I would imagine that extra trains would still be required to "clear" regular trains (at passing sidings).