I am searching for any mod components (or full mods) that made changes to diplomacy options, either by altering the AI behaviors in choosing diplomacy actions, or reacting to diplomatic actions; or that add any new options to diplomacy. Any suggestions?

If you look in XML/Civilizations/CIV4leaderheadinfos.xml
you can change the parameters for when people will be willing to trade with you, and what change in their attitude will result from, say, refusing to help them in a war, or for giving them a tech, etc.

It's kinda tedious though, because it's different for each leader.

New diplomatic options cannot be added currently. They're hard-coded in the EXE.