There's really not much to it. The game already HAS the "Always Peace" and "Always War" options, they're just disabled in the setup menu.
All you need to do is edit the options table in an XML mod, like this:
Code:
<GameData>
<GameOptions>
<Update>
<Set Visible="1"/>
<Where Type="GAMEOPTION_ALWAYS_WAR"/>
</Update>
</GameOptions>
</GameData>
Do that, and "Always War" should appear in the Advanced Setup menu under the Game Options list automatically. It's a boolean, defaulting to "true", but that option set it to false with a zero, and setting something in XML to "true" doesn't always work right. But you could try 'Set Visible="true"' if the above doesn't work.
Other options that have been disabled this way:
> Always Peace
> No changing War/Peace (i.e., locked relations)
> Lock Mods
> No Science
> No Policies
> No Happiness
> No Tutorial
Most of those were obviously implemented for scenarios, but the point is, you can turn them all on easily enough if you want to use them. In my own mod, I do the reverse and disable a couple options (like Quick Combat) because of conflicts they have with certain Lua events.