Here is the link to the
Alien Gear Cloak Dock Holster Mount.
I do not know about the legality in Virginia. In most states, if you have a carry permit and open carry is not prohibited, this shouldn't be an issue. It only can become an issue if there is a specific statue that prohibits mounting a gun to a car (which I don't think exists), or, which is more likely, if open carry laws or laws about access to guns (such as a passenger who is under age or does not have a carry permit) can complicate things.
As for LEOs, they will follow their training. Virginia does not have the duty to inform, but a handgun mounted in a place that the officer can see immediately or at some point during a normal traffic stop bears rethinking the strategy of not informing the officer (which is my default).
As for usefulness, this has been discussed a bit already on this forum. It requires unnecessary gun handling, which should be avoided. If you are in a parking lot that is full of people, are you going to move the gun from the car mount to your holster or are you leaving the gun in the car? Whatever you do, your gun becomes visible to those around you. They may not notice, but you having a gun in your hand while being in your own car can cause a panic if seen by someone outside.
If you have to leave your car quickly, such as after an accident or just because you forgot to buy something at a shop, you are more likely to forget your gun in the car. This not only opens you up to a quick smash and grab job, but also to not carrying your gun when you should.
I do not know how well this product is designed. In a car accident, your vehicle-mounted gun can easily become airborne and lost in the car, which is especially bad if you need your gun after the accident. I previously recommended to screw a BlackHawk Serpa holster or a Safariland holster, both with excellent active retention, to the car.
Alternatively, you can learn how to draw your firearm from your OWB holster while sitting in your car. Every LEO has to learn this, so could you.
Just because there is a lot of gear out there that one can buy, that doesn't mean one should buy it. It just means that there is a market out there for the product. Mindset, skill and gear are the important factors in a gunfight. Most people ignore the first two and go all out nuts on the latest, greatest gear they can buy.