A possible answer to the problem of having a map border taking you to another map is that the code is already there for it. (This follows Dwip's "messy" answer but I don't see any other way personally without massive changes to the overland code.) I found the following while looking through the overland.c file:
if( sector == SECT_EXIT )
ROOM_INDEX_DATA *toroom = NULL;
enter = check_entrance( map, x, y );
if( enter != NULL && !IS_PLR_FLAG( ch, PLR_MAPEDIT ) )
if( enter->tomap != -1 ) /* Means exit goes to another map */
enter_map( ch, NULL, enter->therex, enter->therey, enter->tomap );
I am assuming that "if( enter->tomap != -1)" means that if you set an entrance at each map border then make it's reference number anything other than -1 then you can, in the area file, tell the overland code which map to end up on.
I am not as familiar with the code as I would like to be so someone correct me if I am wrong. I can not, for some reason, find the reference to the enter code that I know I saw when looking through the docs. I will post that reference if/when I see it again unless someone else beats me to it.