From a quick google search, Disney has 30,000 hotel rooms on-site. Let's take that and factor in quadruple occupancy for every room, so 120,000 guests. Now let's say the capacity of the land is the 15,000 mentioned early. 120,000/15,000 = 8, so we need 8 complete rotations of crowds within the land to cycle through that amount of guests. Now, you could do 3 hour rotations and get through that amount in a 24-hour period, but that's not a practical solution. I think a better solution would be to limit the reservation for on-site guests to a single 4-hour window per week (so for those staying longer, they can experience it more than once).
The average Disney World stay (from what I could find online) is 4-5 days, so let's say worst-case-scenario the hotel rooms completely cycle twice in a week, that's now 240,000 guests with 16 rotations of the land needed. Factoring 4 rotations/day of the land, that's 28 rotations for the week, meaning just under half of the reservations should still be available for other guests.
I think as long as you limit the on-site reservations to one/week for each room, it would be practical for them to use the same system. This is also assuming 100% of guests staying on-site will use their reservation.