If you want, you could call James River Bus Lines, who operates it, and see if they would be willing to sell you a ticket on the spot in a pinch.
While it is true that the bus is owned and operated by James River Bus Lines, it would do no good to call them, as they are contracted by Amtrak to provide this Thruway bus service. You would have to contact Amtrak.
This would be similar (although opposite) to you calling the President of Toyota to buy a car in your home town. Even though your local dealer is a Toyota dealer, would you call Toyota HQ?
Besides, if (for some reason) James River Bus Lines wrote you the ticket:
- You would not get the 100 AGR points for the Thruway bus, and
- If you had a sleeper out of ALX, you may find it sold as an on board upgrade, since you did not board at ALX and Amtrak did not know you would board down the line. Thus when the train gets to RVR, you may find that you have no sleeper, and if the train is sold out, no coach seat either!