There are already addons that do this, although I can't think of the names off the top of my head. Look in the chat mods section here.
If you want to do it yourself, you probably need to hook ChatFrameEditBox's OnTextChanged and OnShow handlers, and actually modify its OnEnterPressed handler... (
http://www.wowwiki.com/UIHANDLER_OnTextChanged)
i.e. ChatFrameEditBox:HookScript("OnTextChanged", myFunction)
etc...
To do the sticky channel stuff and also watch for: 1. other sticky channel changes so you can disable your override, 2. anything else starting with / so you can ignore it.