Non-modded server support?

Hi,

I want to create a mod which would work on servers that are not modded (and don't have the mod installed on it) - So far whilst playing around, the only thing I can get to work is an overlay. I can't get anything else to work, even a simple message sent to the player client side. Is it not possible to create something like this? I'm aware of the limitations of this kind of project, but I basically want to create a mod which basically just adds some macros, for example getting the player to put "x message" in chat when they press "y button" on the keyboard - I hope that makes sense?

Thanks