I don't recommend it, unless you have a problem feeder. I think over-eating may be a slight risk, but what might also happen, is that the snake starts to just grab them and then let go of them somewhere outside the dish when it has eaten enough. I've seen it happen with some of my snakes.

How many guppies do you have? With a small population, there is a real risk of the snake exhausting its food supply. Again, we need to use a little math. The increase in biomass in the fish population, must be equal to, or larger than the biomass removed by the snake.