The idea of appending to the list might be the way to go. It's (probably) doable, but it will require work on your part.
What you'd basically need to do is create new Python code that reads your mod's tunable. Then you'd have to find the original GLOBAL_AFFORDANCES list and append to that. Every mod would have its own module tuning, ensuring it wouldn't conflict.
You could even do that on an object-by-object basis, but you'd probably need a mapping of object -> interaction list and then modify the original object by appending your new interactions.
I can't really guarantee that this is going to work, since the tuning system is fairly complex, and I don't have the code in front of me right now.
This is just off the top of my head -- I might be overlooking something.
