Is there a $TM_... path that can tell me the top-level directory for the open window?
For a project yes, TM_PROJECT_DIRECTORY
I mean achieve the same result without using "projects".
The file list that shows up when launching mate from the terminal (mate projectdir) is obviously called "project drawer" even if we just opened a directory. Anyway, the list of files there, The top-directory, that is what I want.