In both the Pathfinder games when you are in one of your "hubs" you have permanent access to the inventory of all the recruited companions, active in your party or not.
They also usually mind their own business and you are asked to confirm the party formation only when you are leaving the area.
Simple. effective and it doesn't even require any particularly elaborate custom UI specifically for that goal.
Trust Larian for making simple things unnecessarily impractical/convoluted when it comes to UI, controls, etc.
