I relate it to buying tickets to an MLB game, and buying parking tickets. This is done on the Ticketmaster site, linked from MLB.com. You first select the game tickets and seats. Next screen is an option to purchase parking ticket. If no parking is available, you have the option to continue with tickets only, or cancel altogether. Disney could modify their system to something like this, especially for guests booking rooms and tickets together.
Or, make it even simpler, like Dollywood did last year when they required park passes. Anyone staying on site automatically gets park passes for the length of stay. For WDW, just make it a generic park pass for any of the parks. I'm sure Disney has stats on what percentage of on-site guests visit each park on a given day and can easily use that for staffing plans.