In games in which AMX Mod X will run (Counter-Strike 1.6, Day of Defeat 1.3, etc), there is a server configuration to execute a file on every map load. The cvar is "mapchangecfgfile". To make this work, you would simply set it to "server.cfg":
I'm not sure if this same cvar exists for Source games. Try to see if the setting exists. If you have further questions about a Source server, you should post in the Source server section of the forum
If you need to request a plugin for SourceMod, you should post in the SourceMod section of the forum.