Virtual backgrounds for any videoconferencing or webcam application I assume the answer is no, but does macOS provide a way (or is it feasible via user application software) to enable (force) the use (via software from a single application) of e.g. a custom virtual background for any (all) videoconferencing applications that may access the front camera? 

I assume the answer depends on what means MacOS provides to application developers for managing the video stream. But for example, can one user application in macOS modify the video stream for all other user applications? (e.g. assuming already that macOS does not provide itself an end-user friendly interface to define virtual backgrounds). 

By virtual background I am referring to the ability of customizing the background on a video call so that people can see the person looking at the camera in front of the camera, but the user can use a e.g. static picture or animation for the background. Examples include this feature supported from Microsoft Teams, or this one from Zoom.
The answer is yes. Applications can create a virtual camera, which other applications will have available in exactly the same way they have the built-in camera on a laptop available, or a USB-connected camera for example.

The application would need to read the images from your real camera, insert your virtual background, and then expose it as a virtual camera.

For example see virtual webcams such as: OBS Virtual Camera or Webcamoid.

If you combine the OBS Virtual Camera program with OBS Studio, you can use the green screen (chroma key) features to get the setup you're looking for.