The spell list is per unit (as it was in Gaiden). There's a
vanilla_spell_lists.event included in the repo, and each unit can have up to 5 spells defined there, plus the level learned. There's also a
Tables where you can associate each unit to its spell list pointer.
If the unit's current class has the "Promoted" flag set, it adds 0x80 to their level when calculating learned spells. So you can set promoted-only spells for units by adding 0x80 to the level you want them to learn it, and they'll have access to all their base-level spells when they reach their final class tier.
The unit won't be able to use a spell if they don't have the weapon rank to use it through normal means (and they won't display the spell menu at all if they have no rank in any offensive magic) - so for, say, L'arachel, I set her up to learn both Fire and Lightning as level-1 spells, but she'll only be able to use the corresponding one when she promotes to Mage Knight or Valkyrie respectively.
The process to do this for staves/white magic is pretty similar - I'll also look into adding that option in a future update.