DIY closed loop system

[mike31154]

You could, provided your overflow can handle what the return pump delivers. If not, you'll have yourself a flood, followed by an empty overflow, followed by a starved return pump, followed by a seized return pump or tripped circuit breaker, followed by.... you get the idea.

Most overflows are not able to deliver the drain capability for an oversized return pump and too much flow through the sump causes all kinds of other issues. Closed loop is not as limited since no air is involved, it's kind of like a full siphon on steroids.