The brain knows no such distinction.

Of course it does. Otherwise, remembering something would be the same as experiencing it. And remembering a thought would be indistinguishable from having the thought.

But I don't believe that thoughts with negative affect are the only things which trigger avoidance, or even the most common ones, and that is where our positions diverge. The early stages of my procrastination spirals do not include guilt, self-hate or any detectable emotion at all;

Have you actually tested that? Here's how you can test:

At the first moment you put something off, ask yourself, "What happens if I don't do this?" And observe your immediate, unconscious, non-verbal or pre-verbal response.

If you get nothing negative from that, try, "What happens if I do do this?".

My guess: you will rarely need to go to the second question to find a negative response, but if you go to the second one anyway, you will find it surprisingly similar.

The catch of course is that in order to do this you have to be able to catch your non-verbal thoughts as they go by, which can be a tough skill to learn, because everybody thinks they already know the answers, so they never really listen to themselves after they ask the question: they jump straight to making up explanations.

As I said in another recent comment: here are some clues that you're making something up instead of actually listening:

• you're using complex sentence structure or abstract/non-sensory words
• the answer is something you "already knew" or expected to find
• the answer makes you look good in some way (either by deflecting blame or trying to sound humble "i.e. poor stupid me" explanations)
• any verbal portion of the answer is longer than a slogan or proverb shouted in anger or other intense emotional state
• the answer is unemotional

The degree to which each of these suggests a made-up answer varies, but if you're hitting two or more of these points, you're almost certainly fooling yourself. The real machinery that runs your brain is non-verbal (feelings, images, and sounds), emotional, and fast (brief flickers of images, short sounds, and somewhat longer flinches and feelings). Most of the rest of our thought process is just verbalizing about those other bits, other verbalizations, or just making shit up.

Mostly making shit up.