Any way to modify DISPLAY similar to how it works for X11 with the macOS display system / window manager so other users can display on active display?
No
X11 includes the ability to run an application on once machine with display on another.
For macOS the application and display must be the same machine.
What can be done is use screen sharing so that if you share B's screen to A. A can interact with the program running on A's machine.
There are also applications that allow you to broadcast from one machine to another - e.g. if running on B's machine, other machines can see B's interaction with the program.