The Rolling ball / Sliding paraboloid algorithm is a well know singular background subtraction approach. It seems this would be a worthwhile addition to OpenCV. This is a fundamental and potentially very powerful approach.
In case it helps, I've already have some work in progress on this, though lack the full insight to integrate this appropriately into OpenCV. I'm happy to share this. Additional links below.
ImageJ / FIJI Java version: https://imagej.net/Rolling_Ball_Background_Subtraction
A python implemention making some use of OpenCV: https://github.com/mbalatsko/opencv-rolling-ball
Potential approximations: https://stackoverflow.com/questions/29320954/rolling-ball-background-subtraction-algorithm-for-opencv