@David L This requirement is impossible. Even if you could send every single email at the same time, there is no guarantee that the recipients' servers will all accept them at that time.
But you can't send every single email literally at once because that technical model would decimate Marketo's (and any mass emailer's) business model. The service must be allowed to amortize resource utilization to a reasonable degree. If you run across a multi-tenant service that guarantees that every recipient in a massive list will be attempted at once, regardless of the list size and regardless of resources in use by other tenants... that service is a scam.
If you are running a contest, start the contest a couple hours after the send begins. You can have people RSVP (via click) as they get emails and start an "exciting" countdown timer for them, but don't expect to start the contest in 5 minutes. That's not how email works.