The D:OS editor supports plugins actually. However, the list of loaded plugins is hardcoded and you need some C# magick to make it load external ones. Also, the native (C++) stuff is not exposed in any header file, so making calls to it is harder.

The startup code looks something like this:

pluginService.AddPlugin(new TerrainPlugin());
pluginService.AddPlugin(new AIPlugin());
pluginService.AddPlugin(new CharactersPlugin());
pluginService.AddPlugin(new ItemsPlugin());