I've made some progress on getting #MonoGame running on the hell distro! The surprise exit-without-error was a segfault (thanks strace!), which apparently was because #NixOS cleverly hid away udev, which is famously an optional component in all modern Linux distros. Now with that fixed, it simply cannot OpenGL:
"Unhandled exception. Microsoft.Xna.Framework.Graphics.NoSuitableGraphicsDeviceException: Failed to create graphics device!"
(fails somewhere in MonoGame.OpenGL.GL.LoadExtensions)