Could we then convert the output of the V2L to DC (using a voltage regulator to smooth it out) and connect it to a string using a PLC - so when solar voltage drops below 50VDC, the PLC would then allow the DC converted V2L to feed that string.
Drops below 50 VDC? Oh I see, if over 50 V, there is no need to use power from the car, as it's presumably day time. But then you have the problem of potentially turning the panels into giant infra-red LEDs. You can solve that by disconnecting the panels, but that requires a moderately expensive DC rated contactor or relay.
This is a common idea, but there are problems. Firstly, converting AC to DC, especially single phase, is not easy at high power levels. Sure you can just use a bridge rectifier and capacitor, but then the power factor is very poor, with current flowing only near the peaks of the sine waves. The peak current is therefore very high (10x to 100x the RMS current), and this causes heating in the cables. You would have to reduce the power by 10x or 100x to prevent the V2L system from exceeding rated current, and the initial turn-on would have to be managed carefully.
The proper way to do this is with a boost converter, commonly known as a Power Factor Correction circuit. But these are expensive and not common off the shelf.
Then there is the problem that the inverter is expecting a solar panel, with a very soft voltage versus current curve. Pure DC out of this sort of system has a very sharp curve. You can soften the curve by adding resistance, but this wastes power, and the resistor would have to be enormous.
There is the safety problem: high power, high voltage DC is dangerous; you really don't want home-made gear dealing with this.
Finally, there is the problem of keeping the car in V2L mode for long periods of time.
Overall, it's a neat idea, but the technical details make it impractical.