There is actually already a mechanism in the game like that.

A mechanism where a companion does not hand out a certain item.
Give Astarion the Necromancy book you can find in the destroyed City... "somewhere" °cough°.
He wont hand it back to you actually! grin
You have to send it to the "camp"(?) and then loot it from the inventory chest there.

Lae'zel should absolutely not hand out her armor to anybody.
Not even to the our OC with the best reputation towards her.
After all this Armor is proof of everything she takes pride in.