Units getting a combat bonus / free promotion for X turns after backstabbing...

[Toggaf]

...meaning declaring war on a civilization you have a declaration of friendship with?

How would one get started implementing something like this, or is there some example code from a current custom civ that I could refer to that does something similar?

 

[Enginseer]

Nope. There's no LUA snippet to detect backstabbing to my best knowledge, so you'll have to resort to these snippets. Basically, you have to create a LUA script that detects the proper civilization and then act on that proper civilization when he has committed a diplomatic action then temporary grant a free dummy "Statue of Zeus" in the capital that gives you a promotion throughout your empire of what you want on and then despawn the dummy wonder to which the promotions SHOULD disappear.