What about creating a Reading Drive Chance Bag and put the really rare, unbuyable and past donation books in there.

Aside from the fact that they just released a whole line of cash shop books, there's also the Lumineve gift, which already has past donation items inside. Plus, if you're someone who has already purchased any rare/unbuyable/past donation books you run the risk of spending csc for a book you already bought.

Then they would surely no longer be, "really rare and unbuyable" Seems like a waste to me, especially since the RD is nearly over.