My guess...just a guess here... The OP is wanting to fly in the day of the cruise and wants the slightly extra comfort that if
DCL books it they'll be aware of changes.
Just plan for an extra day and fly the day before - then book the flight yourself using points.
I've only had them book my flight once - for the day before and not on points - and the only reason I did that was I was changing a cruise from August to May and I'd already paid the August one off. The May one was less and I was able to get my flights and my night before at WDW booked through DCL with the money I'd spent already. In that case it just made sense.