That is exactly how they do that over here and in many other countries as well. So yes, it can be done and it does help to keep cars stay as clean as when they left the factory.
I am not saying it can't be done. I'm saying it's not cost effective, nor efficient, nor reliable. It's burdensome on the people who must get the annual testing and it is infinitely more difficult to mandate, maintain and regulate tens of millions of emissions devices in the hands of the public than to do the same on a large scale at the point of production. Not to mention a massive disparity in cost. It's like suggesting we have each person build and maintain the piece of roadway in front of their house vs. doing it on a large and centralized scale
BTW the Volt needs 225Wh per km so per your examples it produces 116grams of CO2 per km in the UK and 139 grams per km in the US (average). Ofcourse my 14 year old car is less efficient. Its ICE technology from the 90's! I could cheat though and put 100% bio-diesel in my car lowering its CO2 output to 50 grams per km. All it takes for me is to get in my car and drive to a gas station selling bio-diesell about 5km from my home (its just not along the routes I normally drive).
The Volt will go 40 miles (64.4km) on a charge, which is 12.8kW (80% of a 16kWh battery). That's just under 200Wh per km. My example shows that in the UK, it's 1551kg of CO2 for 3300kWh of power = 0.47 grams per watt-hour. That would be 94 grams of CO2 per kilometer according to average UK CO2 pollution rates. If you look at the data I posted, there are no petrol engine cars anywhere close to that number, and only a handful of diesels (like the Smart and Clio Eco) are "greener". But a Smart diesel eco model is not comparable to a Chevy Volt in terms of size, amenities, etc.
So it proves what I said. The Volt is "greener" than virtually every car on the road, unless you take a particularly dirty area for electricity generation and compare it with a particularly clean car.
For most British people, the Volt is going to be much cleaner than just about every other car on the road, with the exception of a few micro-sized diesels that are greener, but not comparable vehicles in terms of size and features. Or, more bluntly, you can't buy a car that is comparable to a Volt (in size/amenities) that is any greener.
And making the equation even worse, is that we are talking about CO2 emissions from the combustion cars alone. It does not include the additional CO2 pollution caused by mining the fossil fuels, refining them, transporting them to the gas station, etc. All of which makes it and even worse case for combustion engine cars.
And perhaps coal/gas power plants are more efficient by themselves there is still a large amount of losses. From the plant to the wheel you have transport, conversion, battery, conversion and motor losses: 0.92 * 0.95 * 0.90 * 0.90 (Prius drive train data) * 0.95 = 0.68. So that is 32% loss between the power station and the wheels. If the power plant has a remarkable efficiency of 60% then the overall efficiency is still only 41%. The problem with electricity is that it needs to be transported and converted several times. While each 'link' is efficient in itself (and thus hard to improve!) the whole chain is not very efficient.
Your efficiency numbers are way off. First of all, from generation to the outlet, it's about 7%. Secondly, the efficiency of generation->outlet is getting better every year. Normal coal plants were 30% efficient years ago. 45% was the standard for newer plants, and the newest are over 60% efficient. And that's coal.
http://www02.abb.com/global/seitp/seitp202.nsf/c71c66c1f02e6575c125711f004660e6/64cee3203250d1b7c12572c8003b2b48/$file/energy+efficiency+in+the+power+grid.pdf
And trying to "work back" into the numbers by inflating all the losses along the way is disingenuous, especially when you minimize all the numbers on the combustion engine side. The data is already out there, and the truth is your 41% number is about 40% low. The real number is that EV's turn 59-62% of grid power into motive force (source: US Dept of Energy
http://www.fueleconomy.gov/feg/evtech.shtml). And in a combustion engine? 17-21%. And don't talk about 40% efficient diesels... a number achieved in a specific engine at a specific RPM are irrelevant, unless you drive at 1800RPM in a direct-injection diesel all the time... what matters is the actual measured efficiency, and that is 17-21%, vs 59-62% for EV's.
And why is it that people love to talk about transmission and generation losses for electricity, but willfully ignore the cost of drilling for oil, refining it, transporting it to the gas station? Because they aren't looking to understand the numbers, they are looking to further an agenda.
Fact is, EV's are much greener than almost all combustion engine cars, with the exceptions being particularly efficient diesel cars (the quantity of which you can pretty much count on your fingers), and/or people who live in particularly dirty areas for power generation - and even then, the comparisons don't hold up unless you compare well appointed cars like the Tesla/Ampera to econo-boxes like the Smart and Clio - hardly a fair comparison.