I was able to attend the Battery Day event, so I scoffed at Elon alone in my car when he made the statement that the Roadster could do V2G but nobody wanted it. As
@AEdennis mentioned a few posts earlier, this capability derived from the AC Propulsion drivetrain upon which the Roadster drivetrain was based. Tom Gage, then President of AC Propulsion, told me that their AC 150 controller (PEM) could do V2G. See
here also. The initial design for the Roadster was to use the AC Propulsion controller, but it was an analog device that was hard to build and maintain. JB Straubel and his team at Tesla created a digital design with the same functionality to replace the AC Propulsion unit. I never heard a direct statement from Tesla that the V2G capability was retained, but I assumed so. That's because Alan Cocconi's Reductive Charger design in the AC 150 was inherently bidirectional. Tesla licensed that design for Roadster 1.5 (see the license label on the left end of the PEM) but made a new design with a separate charging circuit for the 2.x cars to avoid the license.
My home solar PV inverter with battery backup is similarly bidirectional. It can pull power from the DC bus where the batteries and solar panels are connected, lowering the DC voltage, and push it to the grid by raising the AC voltage above what the grid is trying to provide. That makes the meter run backwards. Or it can pull power from the grid to recharge the batteries, lowering the AC voltage (because of the load) and raising the DC voltage.
The car could do the same. No additional plumbing is needed, but different firmware definitely would be. However, as Elon correctly said as part of his answer to the V2G question, this is not sufficient for actual use. You can't legally deliver power from the car to the grid in this manner. If the grid goes down, you'd be trying to power your whole neighborhood. If the problem was local at the transformer that feeds your house, the car might be able to handle the load. But that would energize the home side of the transformer that the utility repair person would expect not to be energized, which could cause a fatal accident.
To avoid this, the home solar inverter has "anti-islanding" protection to detect when the grid power has gone down and stop pushing power to the grid. This happens in a fraction of a second. In order for the battery backup in the solar system to keep power going to the home loads when the grid goes down, the inverter has two AC connections, one to the grid and a second to a subpanel containing breakers for the "critical circuits" in the home, e.g. the refrigerator, lights, etc. When the grid is up and the sun is down, the inverter feeds power from the grid AC connection to the critical circuits AC connection.
So, you might say that to allow the same functionality using V2G would require a second AC connection from the car to the critical circuits subpanel. But you don't want power to have to flow through the car to the critical circuits when the grid is up or the refrigerator would stop when you go to work. So instead that intelligence needs to be in a switching box mounted in the home and a more sophisticated control connection would be required between the car and the EVSE (wall charger).
