There have been systems made to cool fuel, Crytec or something I think they are where called. They use a cold gas to cool the fuel rail to get cooler fuel in to the engine. Its quite expensive though
A cooler in the line wouldnt do the job as well, but it would cost a fraction of the price!
The viscosity of fuel doesnt really change with temperature as it has a very low freezing point, and it evaporates at very low temperatures. Petrol in Liquid form doesnt burn either, its the petrol vapour that burns