Well, turns out, I cannot actually adjust Lightly Armored like that, because putting Options under a feat only leaves you with an Action you can add, rather than a Modifier (for adding Proficiency). No reason to it, that's just how it goes, when nothing is modular. Guess it's time to create separate feats for both.
That did it. Prerequisite Sorcerer + Prerequisite Wizard does act as an either/or for who to show the feat to. Modifier: Proficiency Medium Armor doesn't of course do anything, but baby steps. Edit: aaand actually nothing adds Medium Armor proficiency. So I guess I couldn't have the playtest Lightly Armored in my game even if I wanted to.