I must be misunderstanding, as I think you can. I can click a spell and then either the portrait of a party member on the left to cast it on them, or if in combat the portrait in the combat queue at the top of the screen.
Or were you thinking of the circular portrait down at the bottom for casting a spell on the currently selected character? That I agree doesn't work, and it might be useful if it did.
I stopped clicking the portraits in the combat queue though. There have been instances I've selected an enemy in the combat queue to cast hex or hunters mark, but instead it casts it on the party member whose turn in closest in the queue. It's happened a few times and I was very clearly in range of and selecting the enemy. So I don't trust selecting from the combat queue anymore and would warn others to be wary of it.
