The car charging system would know how much current it is taking and hence power, so would send the meter that value, and that's what it would use for evaluating how much to charge you.
I'm not sure you could do that without being detected.
Might work if you can affod the UPS, I have one here beeping in teh lab at me, they aren't cheap. it'd cost £1000s.
It will need that anyway to regulate the charge.
It really isn't that difficult.
it's already being worked on, and really isnt that difficult. Even the simplest of systems would work, when the meter 'see's a sudden cur rent surge in current as a car battery charging will have that effect, the rest is just simple calcluations.
another method would be to use the car itself. Now the govenrment some how know how much fuel you put it, they can work out the electric too by having a meter in teh car.
But if there are problems the govenrment would just set ther same VAT on al l electicity supplied to your house as it does 'currently' ;-P. It's not like the government will be doing it either they'll but the probl em on the the electic companies.
But it does give me an idea for a student project.