You cannot make a plugin and have it execute a file based on what map it is. The only reason is the command cant be used by the server itself, A client has to execute it because its a metaplugin i think...
So what i was thinking was add ent to a random user thats on the server at any given time, and also execute a es_load "mapname" file... then tell that plugin to stop untill server restart.
Heres the add ent plugin
http://forums.alliedmods.net/showthread.php?p=415157