Opening one of the standard game scenarios in HPS Gettysburg with the editor I found the protection value there at 5 hexes for entry hexes. Not having found one that is both an entry and Objective exit hex I can't say for sure how that would be handled. My guess would be if it's an exit hex the units there would not have protection.
The way to see this is with the editor, open the scenario and go to the Reinforcement Dialog, from the help files
"The Reinforce Dialog is used to define certain data associated with a reinforcement in the current scenario."
" The Protection value is an artificial value introduced into the game to protect the reinforcement from enemy units that are crowding the arrival hex. If the Protection value is non-zero, then at the moment the reinforcement arrives, any enemy units within the number of hexes in the Protection value from the arrival hex are automatically routed. Typically Protection values are used to protect reinforcements arriving on normal roads, while this value is set to 0 for leaders and reinforcements arriving at locations such as river landings (Pittsburgh Landing at Shiloh for example) or across bridges."
I recall playing a blind scenario some time back, my opponent was doing very well with an attack on my weak flank but unknown to him I had reinforcements due there. When they came in several of his units routed allowing me to roll him up instead. Having had the same thing happen to me once it doesn't seem to be a wrong thing to have happen as in both cases the reinforcements arrived on the flank/rear of the other sides units.
Gen. Ken Miller
Army of the Shenandoah