Could maybe make it so that it gets the md5 hash or something of the map file and if it's not correct (doesn't match with the md5 hash or whatever you use of the real correct file preset into the game or something) make it complain or something
I was thinking that same thing at first, but then I remembered an easy way to do it without worrying about that
As I mentioned in chat with DJ yesterday-----
All you have to do is write a script that creates dummy room files. The host of the map is the only one who physically has the external map file. When he starts a match with that file, a bit of "prematch" goes on (let the users chat or something). During this prematch, that script runs on all of the opponents machines and populates those rooms with tiles and objects. This way the only one with an *external* room is the host, and honestly even on his end that script should run and generate the dummy rooms.
Once the script finishes running, it'll flag the player as *ready*, and once all players are ready, badaboom! match on with no external file being read :p