Will this work? (And can I even do it?)

[Figmentmommy]

No need to remove the tickets. You just ask for a ticketless package when calling to make your resort/campsite reservation.

This! You can book a room only reservation and link outside tickets to it. Then you can call to switch to a ticketless package with the dining plan.
We've never booked a throw away room so I can't speak to that, but we have booked a room only with outside tickets linked - even with fast passes made - and then called to add the dining plan. The reservation was switched to a ticketless package with dining and our tickets/Fastpasses remained as is.