Since you can each book something (2 rooms per household address), you can book Alaska and she will be able to book her choice, which is a placeholder. I don't think it matters that she happens to be on your sailing, as long as you are the lead doing the booking.
I agree with what Dug720 said, you don't have to name her as being on your Alaska cruise. I don't know what happens if you don't, however, then add her and it happens the price changed in the meantime.