[Lazy]

Allright, I have done some basic testing and this plugin works.
What you need to do is create a new directory in your mod root folder called "motd".
Then, in that directory create files called mapname.txt for the maps where you want custom motds.

These work in the exact same way, meaning you can have the contents as a url and it will load a web page or just plain text.
If there is no matching file in the motd directory the default motd pointed to by the "motdfile" cvar will be used.