In principle, a focal reducer does concentrate the image circle, which can increase effective brightness and sometimes improve use of the center of the image. But simply reversing a teleconverter is not a reliable way to get there.

Teleconverters and focal reducers are both specialized optical groups, but they are not generally interchangeable just by flipping them around. A teleconverter is designed for a specific optical role, often matched to particular telephoto lenses, and its quality depends heavily on that intended use. Reversing its elements does not magically turn it into a well-corrected focal reducer.

So: physically possible to experiment with? Maybe. Likely to give good image quality across the frame? Usually no. The community consensus is that repurposing a teleconverter this way would produce highly questionable results, because modern teleconverters are not general-purpose optics.

If you want a real focal reducer, use one designed as such for the lens mount and camera system you’re adapting.