I figured out how knowledge checks fit into the 20 and 1 are magic rule.
If you roll a natural 20 on a knowledge check, you write down on your character sheet that you have expertise in a particular sub-branch of knowledge. From then on, you get a +5 bonus to all such knowledge checks. For instance, if you crit on a knowledge check about the Death God’s sacrificial rites, you write down “expert in Death God rites”.
If you roll a natural 1 on a knowledge check, you write down your ignorance on the subject (“ignorant about Death God rites”.) From then on, you automatically fail on any checks on that subject.
This lets you collect knowledge like treasure, and randomly fills out your character’s interests. It also lets a good or bad Knowledge check inform further ones.