That would be a bit OP wouldn't it, seeing as the 10 Pachinko costs 900 Kobans for just 1 guaranteed girl. Sure you can maybe get another from the other draws but what are the odds of that. Even it will only apply to droppable girls as a battle consumable.. if you can buy multiple of such boosters, then you could just buy the droppables away to get a guaranteed Pachinko exclusive girl from the 10 roll.
I mean, it may sound nice from the player's perspective, but I doubt the devs would think the same way. ^^;
However I do support the idea of having something else worth spending Kobans on because right now the incentive for buying Kobans is pretty low in my opinion; which is not a good thing because I'm sure the devs could use more support on that matter. I may be mistaken though, I don't know; but personally I'm not inclined to buy Kobans.