Here is the interesting thing about the Diadem of the Savant, it counts as both no armor (for the mage armor perk) and as light armor (for the custom fit perk).
I tested this with both my mage and my light armor wearing thief. I suspect this is an oversight by the developers as circlets are supposed to have no armor value, but this one some how ended up with one.
The amulet of articulation that @agf mentioned in the comments falls under the same category, as necklaces typically shouldn't have armor value, but in this case it is ignored by mage armor for the purposes of calculating the bonus.
Another clue that backs this is up that neither of these items can be improved at the workbench, so the game may not consider them to be armor at all.
From testing, I'm guessing that the mage armor perk is looking for specific types of armor rather than for anything with armor value, as I have the Lord's stone, which adds 50 armor value, and ebonyflesh still provides 300 armor rather than 100.
All the masks in the game seems to fall under the category of light armor and is classified as such, but I'm guessing necklaces and circlets are ignored by mage armor even though they may be mis-categorized as light armor (which the light armor perks looks for).
None of this can be confirmed until the CS is out, but I suspect this is most likely true.
As for agf's corollary question. No, the amulet of articulation does not negate the well fitted perk for heavy armor.