I'm working on a project for a client that uses 16:9 source material. They need a 4:3 output.
On a PC Avid Newscutter that is pretty much an impossible task.
The path of least resistance is actually to export 16:9 Same as Source, import into After Effects, and let AE do the math. You export whatever size raster you want.
Amazing that the market leader falls down on something that seems that simple.