I don't know of anyone making gas shutoffs, but assuming you're looking for it to fit the AR-15, you might try checking some of the custom makers. They may make something like that. It shouldn't harm anything. Turning off the gas at the block would do nothing but make the gun a single shot, though it would increase the amount of gas behind the bullet and probably increase velocity slightly, throwing off your sights.
As for a forward assist, the only thing it's really good for is turning a simple feeding malfunction into a full out jam. By trying to force a misfeeding round into the chamber, you can compound the problem. Better to just cycle it out and try again with a fresh one.