Hi Rich:
Yes, this is entirely possible. Here's a a code example of how you could do it, needing the appropriate image and alt text:
<a href="{{member.webinar url}}"><img src="http://example.com/yourbutton.gif" alt="Watch!"></a>This is more on theory/prediction but as an aside:
An interesting "gotcha" in this situation might be that the email link click isn't tracked within Marketo. We just tried out using GoToWebinar and sent out a thank-you email to our registrants using this token, but because (I assume) it's a text token, it isn't filtered by Marketo in a way that replaces the URL with a tracking code. It's just a pure URL unique to the person. More on the
differences between text tokens and rich text tokens here.
Obviously you'll be able to see who the attendees are by other means, but there might be a discrepancy between the number of attendees and the number of people who clicked that link and left. Not having too much experience with WebEx, I couldn't tell you exactly if this will be a problem or not for you.
Hope that helps, Rich!
Best,
Edward UnthankSEO/Web Specialist
Yesler