I thought others might be interested in what the police would accept for a Hungarian resident to be driving a non-Hungarian registered car.
The Hungarian (resident) needs written permission of the owner to drive the car and must have this permission for each day they can drive the car. Each permission must be dated individually and cannot cover a period, only a specific day. Apparently it needs 2 witnesses to sign as well (although that might be a little over the top or precise). However one could quite easily imagine, although of course not condone, an automated mechanism to produce these individually dated and signed permissions several months in advance. I believe they must be signed as originals.
I believe if the cops are having a bad day, they will take the number plates and the fine is about EUR 800 - quite excessive. It's not clear if paying the fine, automatically registers you car in Hungary or it's just a fine with no other action. Presumably the cops maintain a list of stops, ID numbers and car registrations and if there's repetitive instances of stops with the same car/ID, then maybe they'd take further action. But of course, unless they immobilise the car physically, for UK drivers, the number plates are easy to obtain. One could carry a spare set.
I can imagine a considerable number of people on legitimate business in Hungary, Hungarians or otherwise, with multiple addresses being very annoyed at these new road stops.
I do wish the Hungarian authorities would lower the registration tax to a reasonable amount for passenger cars, say, EUR 200 or 300 and leave it at that. The registration scheme they run now is absurd. They need to shift the costs onto the annual tax and link it to vehicle inspections.
The Slovakian registration scheme is obviously one of the major targets. One thing which surprised me is that this sort of thing also goes on between Romania (which also has excessively high registration taxes) and Bulgaria (which has low registration taxes like Slovakia). I thought it was just localised to Hungary and Slovakia.