I really like the sound of this it would retain some of the companions character so i dont feel weird about swapping out for example, astarions gear for something with a higher ac or for an advantage in stealth at camp he would still retain his noble appearance and so so forth with the other characters. I dont think that sleeping in full plate would be easy but if they still wanted protection they could strip down to a lighter armour like ringmail or chainmail to be protected and more comfortable.