I am not sure if this would help you, but I recently created a single landing page with a single form which allows you to register for any of 11 webinars. The way this worked was to create a form which used tokens as drop down selections, create a "master" form fill registration campaign containing token values, and use that registration campaign to pass registrations to each individual webinar based on the value of the token. You could do this with actual dates or tokens. I liked the token approach because I will be able to easily reuse the same form again in the future.
The token used for the drop down selector passes the selection to a custom marketo field I called "webinar session" and the token value is the date. Here's the landing page:
http://support.onvia.com/onvia-guide-update?utm_source=onvia&utm_medium=email&utm_campaign=onlinegui...
t's working pretty well so far. I imagine you could use a custom field to contain a value for true/false is "register for all" and add people to all webinars this way. You might want to turn off the auto-confirmations for the webinars though or people will get a slew of registration confirmations all at once. Not sure if you'd want that.
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