If you're using Eloqua's Simple Email Campaigns function to deliver event registration campaigns, you're going about it the wrong way.
Multi-step campaigns empower you to take maximum advantage of the "automation" in marketing automation. The Single City Event Registration Guide will give you step-by-step instructions and includes screen grabs of campaign canvas layouts.
Taking advantage of the automation you have as an Eloqua customer will enable you to launch your invitations and walk away and complete other tasks - like locking in a great menu for your event.
Eloqua will know who has registered, who has not, issue "final chance" emails and then send email and SMS reminders to those who have registered.
Now you're on the journey to Modern Marketing.