Can you not just use a plated hole with pad and place the center on the board edge? I'm not sure, but I don't think you'll be able to make a half plated hole.
Then again Altium has suprised me before with some of their stuff.
I don't use Altium enough to know for sure, but I know in other CAD you would just make it as you would a plated through hole and center it on the edge. I would put a note on the fab drawing for your intentions or contact them to ask about what note you should put in there.
Hope it helps