I am wanting to build a heavy armor illusionist, but only if, in using the Steed stone, the mage armor perk would work as if you had no armor on. Has anyone done this or tried it? Or just straight up know if it would work?

it doesn't work, the weight value doesn't matter. it's armor rating

It would not work.

All the Steed Stone does is make your armor weightless. It doesn't stop the game from acknowledging that you're still wearing armor (which would be silly).

Doesn't make much sense to stack Alteration with Heavy Armor though, since both skills get you to Armor Cap on their own relatively easily.

you can still take it though. you get some extra armor and nice magic resistance.

That's the Lord Stone.

Thank you, fellow wanderers of the unreal.

Alteration is still quite useful with heavy armor if you want to go that way. In the beginning of the game where your armor is nowhere near the cap, extra 40 or 60 armor is nice. It helps level up alteration and get up to 30% extra passive, slot-free magic resistance. And later you get Paralysis spell if you pursue it further, one of the most OP spells in the game.