This is one area where you are doomed to failure if you tinker with the rules. Simply putting in a panel in the back windows is not enough, you will need to have the back seats removed permanently. In any case I guess you will want to have everything correct for insurance and tax purposes so there is no better place to ask that the relevant Revenue authorities. They don't bite, actually they are very helpful even if they are telling you information you don't want to hear. Who knows they may be able to guide you to a good legal solution.