yes i have thought of this idea too when i saw a petrol pressure washer on ebay,
its a good idea to be sure - especially if petrol i think, ignoring the polution side of things, it would really work well if you had a petrol pressure washer at about 100 psi - which would be about optimum for the average 300gpd ro - the fittings on one of them i saw were standard hoselock so just connect to your outside tap and you could produce RO filtered water much faster. It would have to be the right pressure though, i favour this idea over an electric booster pump.