I think you'll find the answer by combining the suggestions of Jessica Johnson and Linda White.
This sounds to me like more of a "How do I best market the Keynote" rather than just a nuts-and-bolts widget customization question.
First, make your Keynote session INCLUDED in registration. Next, highlight the Keynote session on the main page and sessions page (Agenda) of your registration website.
Use the Cvent Event Management emails to highlight/market the session to potential attendees. And if you DO make the session "Included," then you can use a Session Registration email to remind people 10 or so minutes before it to log in and "don't miss the session!"
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Brenda Ainsburg
Channel Program Manager, Americas Channel
brenda.ainsburg@siemens.comSiemens Aktiengesellschaft
United States
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