as per kristijanH:

"A possibility to add cards from library to inventory with 0 quantity. (When a new set or product comes out I add all cards to inventory (ctr-a F5) then change each one to zero quantity then add the number I have). I do this so I have all cards in my inventory even the ones I don't own.)"

This is kind of exclusive to proxy card support in #97.

It would also provide alternative way to produce missing card lists for mixed inventories - might be worth a shot.