To answer my own question… on Linux there is a flag -RenderOffscreen which, if used, does what I want with no drama at all. I am able to run as follows:
/path/to/UE4Editor /other/path/to/my.uproject -windowed -RenderOffscreen -game
I don’t think -windowed is strictly needed and -game is not needed but I want it. I am on Ubuntu20.04 with Nvidia RTX and Vulkan. It is my understanding that that -RenderOffscreen flag is Vulkan-specific. I have also confirmed that this works on a machine that has no window manager installed at all.