So yes, for your situation the switch requirement isn't ideal.
I think your two best bets (about equally complex) are: to use Tiamat-86's idea of a state that grants the armor type proficiency. You would, as caethyril pointed out, have to use a plugin to keep the state on the actors at all times.
Or, using Yanfly's Equip Restrict, it does allow you to use a skill, so you can make an "armor proficiency" or whatever skill whose usage scope is never, so the player will never see it in any skill lists. The characters you want to use the armor right away will start with the appropriate skill, and you'll grant it to the player character in the event he learns it.
Note that the plugin specifies it won't work if the skill is granted via the trait box in the actor or class, so you'll need to give it to the actors via event command in-game.
So either way you're creating some entries in your database (states or skills) and using a plugin (the Equip Restrict or a permanent/passive state).