Illaoi can proc Grasp on spirits...

She can proc Bandit (the gold income mastery) on spirits as well, makes for a nice 10 or 20 gold for every pull

fervor not proccing is just a weird interaction I don't entirely understand

Fervor doesn't stack on clones, and spirits are coded as clones.

More likely than not they have different specific conditions that aren't listed. Same text but slightly different coding.

Grasp and bandit is probably something along the lines of "damaging an enemy champion with a basic attack" and fervor would be "hitting an enemy with a basic attack" the wording isnt that much different but if you play trading card game's like yugioh (no h8 pls) then you'd know. #MstDoesn'tNegateEffects

The exact wording of grasp makes it seem that way. The wording of bandit and fervor are almost the same though so i have no clue.

Just an inconsistency of the coding. But it might be intentional as i dont want fervor stacking off of the soul. That'd be alittle oppressive and i play illaoi.

Just like how you can proc grasp off of the Wukong clone.

But for a more roundabout answer, fervor stacks off of auto attacks in enemy champs. So simply hitting the spirit will not stack fervor as you aren't actually auto attacking champs but rather deal damage through abilities; which unless you have an ability that applies on hit effects (Ezreal Q, Fizz Q, Irelia Q, etc), abilities won't stack fervor.