as long as the sump is lower than the display tank, I don't see why not. You are still utilizing gravity as the main means of moving water from the tank.
If you incorporate a refugium into the sump and use a little creative design concerning water flow through the sump, no reason why it can't be configured in such a way as to present the refugium section as another display.
That is exactly what I plan on doing with one either internal or on the outside of the main display, but either way it will be completely visible from a portion of the front and the one full side of the tank.
Why always hide the refugium out of site, there can be a lot of interesting things that would do well in a refugium.