+1 - why not, there have been times I wish the inventory menu would stay open

or for the sake of knowing when its full, a small counter. like white text that says "28/30" that is toggled in settings tab for instance

That would work as well, better in that it would take less space.  But the backpack lock would be useful for other things besides capacity.  For instance fighting fire giants you would know how many foods you have left without having to mouse over.