Surely somone must have made a DIY version of this. Take the portable sprayer idea (they have safety valve) + one of those cigarette lighter compressors. Drill hole in sprayer, add schraeder valve. You'd charge it up in the boot of the car on the way to your ride. There will be a safety issue if the safety valve fails, but that must be the same with the dirtworker. Putting it in boot makes it safer if it goes pop. Some clever person could make a cut out for the compressor.
Fair play to dirt worker. Good product, deserves to suceed. I don't think the DIY version will lose them profit as tight wads wont buy anyway. Many who try DIY will give p and buy dirtworker when they wouldn't have in the first place.