I would call Disney's customer service # and ask but it is very likely there would be bus transportation that could take you to Magic Kingdom that day. On most days, there is at least 1 park that is opening at 8AM, and there could very well be bus transportation running for all parks starting around 7AM. I am not sure, but it is possible. I seem to recall doing 8AM reservations in the Magic Kingdom area (whether inside the theme park or at the nearby resorts) while I was staying at far away resorts, including 1 year at AKL, incidentally, and being able to get a bus around 7AM.
I think Disney makes it so it is possible to use their bus transportation to make those early breakfast reservations. Maybe it doesn't always work out perfectly, but I don't recall ever not being able to make an 8AM-ish reservation because the buses weren't running yet and I've gone to Disney in almost every month of the calendar year, too, and I think it always worked out.
Again, there might be some scenarios where it wouldn't work out but Disney has even been known to make an exception for on-site guests and send a bus just for them (it's happened to me). Because when you explain to them that you are a guest and trying to simply take advantage of what Disney has to offer, like Dining reservations, they usually do go out of their way to accommodate you, after all, why make it possible to have such early dining reservations if there is no way for their paying guests staying on property to get to them? An on-site paying Disney guest shouldn't have to uber it to their dining reservation because you are already paying for transportation as part of your package.
It doesn't hurt to tell this to Disney.
