Nice suggestion. You can't use the right click button though, due to it let you pick up objects one at a time. Left click picks up the stack, right click picks up one of a stack.
Currently, the mouse wheel button does nothing, so this would be perfect for it. I was thinking of asking to use it in order to switch between the item rows only to find out that the tab button does that. I then tried to reset the mouse wheel button for the item switch but it wasn't recognized as a valid option.
Maybe there is something else the mouse wheel could be used for...
Another thought. Maybe an item lock would mean that items do not move into a chest when hitting the "add to existing stacks" button as well? I try and keep some items in my inventory but then accidently lose it when hitting that button because that same item is in another chest.
You could also make it so locked items can't be tossed in bins, like tools. I had been saving all the mixed seeds in my inventory for a while now, but they have disappeared. I really have no idea what happened, but I suspect that I may have tossed it in the bin by mistake. Even if that's not the case, this is a nice way to avoid the worry.