Yes, I've actually done that. It's a bit tricky to design, because you have to very carefully compute the currents, voltage changes, and reaction times of the comparator. To keep the variations down, such designs are usually for limited input voltage range and a fixed output voltage.
What you describe is really one form of a pulse-on-demand system, in this case implemented with analog electronics. Pulse on demand has more ripple than something that controls the PWM duty cycle to regulate the output. However, they are simple, inherently stable, easy to analyze, and easy to implement in firmware.
I sometimes use a PIC10F202 with a pulse-on-demand algorithm as a low cost buck converter with a lot of forgiveness. In many applications 50 or 100 mV of ripple is fine. This is especially true when the buck switcher is a pre-regulator feeding an LDO at just above its minimum input voltage. One trick I use a lot with this kind of buck switcher is to use a PNP transistor around the LDO as a comparator to determine when the input is one junction drop above the output. That gives the LDO enough to work with reliably, but not so much to waste a lot of efficiency.
It's often convenient to have a +700 mV rough supply around. You can use it to feed distributed-point-of-use LDOs, and to power things that don't need a highly regulated voltage, like LEDs for example. This keeps the current demand off the LDOs, so they can be small and cheap, like SOT-23 or SOT-89 packages.