Same here. We're using Gamebryo for our engine, and they do not provide editors of any kind. We use their in-app tools and export scripts to export the scene from 3dsmax into *.nif format. What are you looking to find out?
I assume you would throw a few maxscripts together that allow to place entities and such. Als it might be wise customising the UI in a fashion that it hides most of 3dsmax, so that basically only the level relevant primitives and operations are left. probably also a simplified material editor. to just assign quickly…
Create models in separate files, or grouped together in a file, whichever's easiest. Characters in separate files, props together, ground tiles together. Script tool creates proxy of a model, which we then merge and arrange into a master game layout file. Any objects that have behaviors or animation or any kind of change…