You may be out of luck in places without a lot of tide. Typically, ramps end very close to the low tide line. In places where they need to go farther, they usually place pre-poured pieces of concrete to extend the ramp.
I have used the same technique as you many times and it does work quite well. I have even done it on a smooth, gravel ramp succesfully which may be an option if you can find a smooth one and have the traction to pull the boat back out.