> You would need another button for that in your system, I guess

That's what I meant: Place a "combine" button next to the moss, like an arrow pointing to a bow icon, for example.

> I doubt that it would be slower than opening a menu in the bow (the functionality could not be put on "Fire") and selecting explosive arrows there.

What "it" are we talking about here? The combination menu would be as fast as that - and more general, as it could support more possibilities for "combinations" in case we need them.

> Imagine you had a shortcut in the backpack for "move into combination slot" that you can hold down and then click on an item to mark it for crafting: experienced players could produce stuff out of their inventory in a split second

Huh? I am sorry, but I completely fail to understand why you are such a fan of those slots. Drag&Drop counts as equivalent to two clicks in my book, so you are proposing to replace one click with four. I can't remember one game that I liked this type of interface in (the new Monkey Island games and Aquaria come to mind - hated it in both). Is there some upside here that I don't see?