I've updated the map editor for Heroes 6. Now it supports saving and loading maps. Also, the archive contains a standard map from Broken Alliance, named Scenario\Test\prologue.scn.
Link: http://www.multiupload.com/T9ZZ3HOUGZ
Installation - extract to the game directory. Run using run_editor.cmd.

I haven't found a "normal" way to launch custom maps from the game (although I didn't really look). But I discovered that when using the "/presstourdemo" key, the game tries to load the map "\Scenario\Test\prologue.scn" or "\Scenario\Test\Mother Namtaru Press Demo.scn". Therefore, to play your own created map, you need to save it in the editor under one of these two names and launch the game with the /presstourdemo key.
For convenience, the archive also contains a file called run_map.cmd, which already has these keys specified.
So, if anyone wants to, they can start creating their own maps now ;)