As a reminder, register_clcmd() is for client commands which can only be sent by a client so that would never have worked. register_concmd() should catch commands from both the server and the client.
I tried register_srvcmd() on a proper server and it also doesn't hook the say command sent via the server's console. It's possible that the engine handles all known server commands before they will be sent to MetaMod (and therefore AMX Mod X) but I don't know that for sure. I'm not able to hook any other server commands either.
It sounds more like you need to describe what you're trying to do and why because maybe there's an alternative method to achieve your goal.